For a decade-and-a-half, they have been each other's great impediments.
If Peyton Manning had played in another time, Tom Brady might have six Super Bowl rings instead of four. If not for Brady, Manning might have three instead of one. The first quarterback Brady beat in the NFL, on a cloudy September day at the old Foxboro Stadium, was Manning. If Sunday's AFC Championship Game turns out to be Manning's final act, it would only be fitting that Brady plays the lead role.
One is the measuring stick for the other, now and forever.
The great defenders who have played against Manning and Brady can appreciate this better than most. They saw and felt the way these two future Hall of Famers fed off each other, ushering in a golden era of quarterbacking. And they were left in awe by the way Manning and Brady have presided over the NFL.
"It's crazy that every year it comes down to these two guys," five-time All-Pro middle linebacker Brian Urlacher said. "It's so hard to get to this game, and they've done it so many times."
"We've been treated to these guys for the last 15 years—the last decade-and-a-half," two-time All-Pro middle linebacker London Fletcher said. "Think about that.
"I just want to see one more great battle where Peyton can turn back the hands of time and have one of those vintage Manning days."
The 39-year-old version of Manning had the lowest passer rating of his career in 2015. At 67.9, he ranked 34th in the NFL—last among qualified passers. He does not direct the offense like he once did. The durability that once was a trademark has been surrendered, along with many other attributes of youth.
For a while, Manning even experienced what it felt like to sit on the bench and watch. But the Sheriff somehow made it back for one more gunfight.
"A couple of weeks ago, nobody thought this would happen," seven-time All-Pro cornerback Champ Bailey said. "For this game to come about the way it has, to see them for maybe the last time is great. I am just glad I was a part of it and got to see it grow from the beginning to the end."
The first time Bailey played Manning, Manning's Colts beat Bailey's Redskins 24-21, which was the start of a trend. Not counting the final game of the 2004 regular season, when Manning was being rested for the playoffs and threw only two passes, Manning's teams had a 6-1 record against Bailey's.
Bailey said Manning gave his teams more problems than Brady did. And he did most of it with his mind.
"With Peyton, you never could outthink him," Bailey said. "He was always one step ahead. His preparation was probably the best I've ever seen."
Manning is less likely to outwit an opponent now that he is playing in Gary Kubiak's more restrictive offense.
"He does some little things at the line of scrimmage now, but in his prime, he was in total command," Bailey said. "He was so deceptive and knew how to trick you. The language, to hear the different words come out, it was amazing. We used to try to get tips about what was coming, but he would change everything."
Most defenders from the Manning-Brady era have at least one story about getting burned by Manning's anticipatory skills. One memory from a Sunday night in 2010 against Manning's Colts still haunts Fletcher.
Throughout the game, Fletcher was countering Manning's checks and audibles with pre-snap adjustments of his own. The Redskins were doing well until a third-quarter play when they threw an exotic front at Manning. He audibled. Fletcher thought about changing the defense, then decided against it. Manning had switched from pass to run, and Joseph Addai took off for a 46-yard run that changed the game.
"If he knew what you were doing before the snap, he picked you apart," said Fletcher, now an analyst for CBS Sports.
Urlacher had a similar experience in Super Bowl XLI. Manning noticed Bears rookie safety Danieal Manning was misaligned on one first-quarter play. On the next play, Danieal Manning was misaligned again, and Peyton capitalized with a 53-yard touchdown pass to Reggie Wayne.
"You screw up against Peyton and he's going to make you pay for it," Urlacher said.
Both Urlacher and four-time All-Pro defensive end Jason Taylor said their teams blitzed Brady more, and played more conservatively against Manning. The point was to keep it as simple as possible against Manning so there wouldn't be a crack for him to turn into a crevasse.
Brady once lost a high-stakes chess match to Bailey in a playoff game in the 2005 postseason. The Broncos were being aggressive, blitzing Brady and playing man-to-man zero coverage. As the game went on, Brady—as he will do—began picking up on when the blitzes were coming. His solution was to call for crossing routes to try to pick off Bailey and fellow cornerback Darrent Williams, which would create separation to allow quick throws.
In the third quarter, Bailey told Williams that if Brady called for a crossing route again, they should stay home instead of trying to cross with the receivers and risk getting picked.
"He never thought I would be hanging out," Bailey said. "He threw the ball up thinking nobody was there. I jumped right in front of him and went 101 yards."
Bailey's big play aside, finding defensive weaknesses probably is an underrated aspect of Brady's game.
"You might hit Tom a couple times, maybe get a sack," Taylor said. "But he will take that mental picture and get you later in the game."
The defining intangible for Brady, however, may be toughness. Taylor recalled a 2004 Monday night game in South Florida. Wearing their orange jerseys, the Dolphins got after Brady pretty good. One hit after another, they kept putting him on his back.
"Some of those hits were good rushes; some were him being willing to stand in there and hold the ball for another half a second to wait for someone to come open," said Taylor, who had 16.5 combined sacks of Brady and Manning. "At times, you'd be like, 'Damn, I thought I blasted him.' But he curses you out, pops up and goes back to the huddle every time. That was somewhat discouraging. I respect that about him."
Manning, Taylor said, was more inclined to get the ball out quickly and avoid the beating, which explains why he's been sacked 115 times fewer than Brady.
"Over his career, he has been more cognizant of taking hits," Taylor said.
That is not a revelation about a lack of courage, though. Taylor remembers when he and Lorenzo Bromell clocked Manning under his chin in a 2001 game, causing a hairline fracture of his jaw. He also remembers Manning, blood dripping from his mouth, coming back on the field after missing just one play.
Neither quarterback has run away from many defenders, but both have avoided plenty of them.
"For two guys who can't move, they are amazing at feeling pressure without looking at it and having the ability to move around in a phone booth," Taylor said. "They both have the ability to make you miss and keep the ball in throwing position, then throwing it that split second before you hit them."
Urlacher was the victim of perhaps Brady's most famous run. Facing a 3rd-and-9 in a 2006 game, Brady took off up the middle only to encounter Urlacher in the middle of the field. Brady, who ran a 5.28 40-yard dash at the 2000 combine, stepped left, then moved right to pick up the first down.
"He juked me out of my shoes," Urlacher said. "I thought…[he] was going to slide, and he didn't slide."
Taylor still thinks about a similar humbling midfield experience against Manning. After a fumble recovery in a 2006 game, Taylor looked up and saw nothing between him and the end zone but Manning. "I could have and should have scored," he said. "I'm not giving him any damn credit. It was a bad run by me, and I'm still pissed he tackled me."
It's interesting that while Brady and Manning are considered among the best ever, neither is considered among the most physically talented. The most remarkable aspect of their throwing abilities, great defenders agree, is their accuracy.
"Tom has perfected his throwing motion over the years," Taylor said. "He is throwing the ball over the top and in the slot better than he was 10 years ago, which is mind-blowing to me. Peyton has changed a little with his injuries and has had to redevelop velocity on the ball, but Tom has been unwavering. It's been amazing to watch."
Manning and Brady have also both benefited from their circumstances, of course. Manning played 20 combined years with two wide receivers who might be Hall of Famers in Marvin Harrison and Reggie Wayne. Brady has played for a coach who is strategically without peer in Bill Belichick.
"It's been Belichick and Brady versus Manning—not just Brady versus Manning," Urlacher said. "That kind of gets overlooked in the Manning-Brady talk."
Subsequently, their offensive systems have been very different.
"Every time I played Peyton he had Hall of Fame receivers, so it wasn't just like I had to stop Peyton," Bailey said. "I had to stop them too. But it was the system that got Tom's guys open against me."
All of which quite nicely frames the debate about which has been greater. Most numbers point to Brady, but it is not as simple as an 11-5 head-to-head record favoring Brady, or four rings to one, or a 22-8 postseason record to 12-13, or a .767 winning percentage to .683.
"Manning gets unfair criticism for those playoff losses," Fletcher said. "Peyton was the sole reason his teams were getting to the level they did. He had to do everything. He had to call the plays. He was the offensive coordinator. And the defenses weren't necessarily great in Indy. But with Tom, it's the scheme, it's the Patriots, it's Belichick. More people played a part in that success."
Whether or not Manning ever is voted king, he may be voted Mr. Congeniality. Manning has offered many of his longtime opponents an autographed helmet in exchange for one of theirs. When Fletcher retired, he received a handwritten note from Manning telling him how much he enjoyed competing against him. And then Manning even named a play in Fletcher's honor.
Brady is perceived as having a thornier personality. He talks a little trash and isn't shy about making loud objections to officials.
Urlacher said if he had to lose his only Super Bowl appearance, he's glad he lost it to Manning. He has that kind of respect and admiration for the way Manning has performed and the way he has handled himself. But Urlacher understands why many consider Brady the best.
"You have to go with the rings with Tom," he said. "Having said that, Peyton has had different head coaches. Tom has had the same situation the whole time. Tom Brady is great, but I feel the Patriots' success is based on their team—the way their defense plays and special teams too. He's had better teams than Peyton. Peyton's success is based more on what he's done, because he hasn't had as much around him."
Comparisons are easily drawn between Brady and Manning because they have been on the same field so often. This is a rarity among top-tier quarterbacks.
John Elway and Dan Marino met only three times in 16 shared seasons. For the most part, Marino and Joe Montana existed in different conferences and on different coasts. Montana and Elway had few memorable battles, and their lone Super Bowl faceoff ended in a 55-10 victory for Montana's 49ers.
Otto Graham ruled the late 1940s and early '50s without peer. Johnny Unitas and Bart Starr faced one another 13 times but had just one 300-yard passing performance between them in those games. Manning and Brady have combined for 14 games against one another in which one of them threw for 300 yards.
You could write a book about these two passers. In fact, Gary Myers has. In Brady vs. Manning, former quarterback Tim Hasselbeck said he would pick Manning over Brady. Hall of Fame defensive end Michael Strahan said he feared Brady more, and Montana said he would take Brady over Manning.
Said Taylor, "Tom is my horse, even if he never wins another race, as far as the quarterback position goes. But you can't go wrong with either one."
There might not be a definitive answer to the question of who is greatest. It is their combined greatness that has defined football for a decade-and-a-half.
On Sunday, we should all appreciate watching that combined greatness for perhaps the final time.
Dan Pompei covers the NFL for Bleacher Report. Follow Dan on Twitter @danpompei.