What's the Cause of Serena Williams' Recent Woes on the WTA Tour?

Serena Williams remains without a title in 2016. She's played in only three tournaments this season and has left each without the winning trophy.

An 11-3 record and second on the season's points leaderboard would be a stellar start for anyone else, except Williams. Her latest loss has many wondering what's going on.

After losing the first set in a tiebreaker, Svetlana Kuznetsova came back to defeat Williams 6-7(3), 6-1, 6-2 in the fourth round of the Miami Open. It was Williams' first loss at Key Biscayne since 2012. A winner at the Miami Open in 2006, Kuznetsova moved into the quarterfinals for the first time since 2009.

In her post-match press conference, Williams told reporters, per the Associated Press (via ESPN.com), "I can't win every match. These players come out and play me like they've never played before in their lives. I have to be 300 percent every day."

Indeed, when players defeat Williams, it's perceived as a monumental achievement. Kuznetsova told Nina Pantic of Tennis.com, "I have so many people saying congratulations, I feel like I won the title already."

Lynne Sladky/Associated Press

A two-time Grand Slam champion, Kuznetsova played a magnificent match, throwing in stealth drop shots as well as hitting winners while patrolling the baseline. Still, the focus is on Williams and her un-champion-like 55 unforced errors and nine double faults.

Williams loses so rarely that no matter how well her opponent plays, the questions are more about her performance.

It's only March, and Williams has already lost as many matches as she did in all of 2015. She's struggled with her signature shot, the serve. She's also looked distressed on the court more often.

As Pantic wrote, despite having won in Miami eight times, "not all seemed merry for the 34-year-old from the very start of what she calls her home tournament."

Williams had to fight off a tenacious Christina McHale in her opening round. She also had trouble holding her serve against Zarina Diyas.

Don't blame the heat. Williams brushed off any ideas about wilting in the record 91-degree weather. A Florida resident who lives in nearby Palm Beach Gardens, Williams told reporters, via Andy Kent of the New York Times, she's used to the heat.

You can also rule out injury. After her loss, Williams said she felt fine physically and wasn't suffering from any injuries.

Her ankles were wrapped tight, but they have been for every match in the past few years. At times, Williams seemed slow to react, but that's usually more the byproduct of motivation and focus rather than injury.

Lack of motivation may be an issue. When you're chasing a historic Grand Slam total, how excited can you get about winning a ninth Miami title? As Williams stated, players are going to enter the match motivated to beat her. How determined is she to beat them?

Lately, Williams appears unhappy out there. Wincing and whining on the court, she seems as if she wants to be anywhere but out there playing.

But you can't say she isn't fighting out there. She had to pull out a three-setter against McHale. She was down 1-4 in the final set against Azarenka and nearly pulled even.

Perhaps Williams has slipped into a confidence crisis.

It may seem crazy for someone with 21 Grand Slams to struggle with confidence. But last year, 14-time Grand Slam champion Rafael Nadal admitted to struggling.

Heading into the Paris Masters in November 2015, Nadal said, according to the AP (via ESPN.com), "It was not physical, it was mental. If I was not enjoying on court and not feeling competitive, so it would probably not make sense to play on the toughest surface for me. I play to enjoy [myself] and to be 100 percent competitive."

Maybe doubt trickled into Williams' game after the heartbreaking loss in the semifinals of the 2015 U.S. Open. Known as a closer, Williams seems less invincible these days.

In her loss against Victoria Azarenka in the finals at Indian Wells, Williams had two break-point chances to level the match at 5-5 in the second set. However, she was unable to convert and ended the match on two forehand errors.

Williams was 5-17 in break-point chances against McHale. In three matches in Miami, she served a total of 16 double faults. Her once-reliable serve has been anything but lately.

Maybe she's been so good for so long that expectations are just unreasonable. When asked about her recent success against Williams, Australian Open champion Angelique Kerber reminded the Miami Herald's Sandra Harwitt that despite that win, she's 2-5 against Williams.

Kerber added: "It's always tough when she's playing great because Serena is the best player, the No. 1 of the world, and everybody is expecting her to win. But I think it's tough for her, too, because all the pressure is always on her."

So without any obvious physical ailments or noticeable mental lapses, the most obvious reason for Williams' recent woes is age.

Williams is aging, and Father Time is undefeated. He catches up with even the greatest. Even Kuznetsova—who in her post-match interview told the stadium crowd in Miami she's "too old,"—is four years younger than Williams.

Nobody can stay at the top forever. Sometimes, decline is shocking and obvious, like with NFL quarterback Peyton Manning, who went from a record-breaking years in 2013 and 2014 to looking as if he could barely toss a ball 10 yards.

Instead, a tennis player's decline is more subtle. A little lack of pace here, a step slower there, and suddenly, the once-dominant star looks more beatable.

Expect to see the more vulnerable Williams more often. Still, she knows better than anyone that she's in the final stage of her career. Her quest is probably for a couple more Grand Slam titles before she checks out.

Watching Williams decline as competitors rise against her may be painful for fans. However, aging is inevitable.