The unseeded Dimitrov beat the 20-time major winner 3-6, 6-4, 3-6, 6-4, 6-2 at Arthur Ashe Stadium.
The 28-year-old was impressive all night, perhaps never more so than when he was up 5-4 in the fourth set. Once there, Dimitrov warded off a triple break point (and five break points total) to tie the match at two sets apiece.
After that, Federer took a medical timeout for an undisclosed reason for close to 10 minutes. He returned to the court but was clearly not the same player, as Jose Morgado of the Diario Record noted:
After the match, Federer revealed that is dealing with an upper back ailment but gave credit to his opponent for the win, per Ben Rothenberg of the New York Times:
While Federer may have played at less than 100 percent, Dimitrov's win was well-deserved considering his sensational return game, historical record against his opponent prior to Tuesday and rough recent stretch.
Dimitrov won 48 percent of his second return points (27-of-56) compared to 30 percent (12-of-40) for Federer. He also had just 41 unforced errors to Federer's 60.
He dominated in the fifth and final set, breaking Federer twice en route to a 4-0 lead and an eventual 6-2 victory.
Dimitrov also won one more set Tuesday than he had against Federer in seven lifetime matches, all of which were losses.
Lastly, Dimitrov was not enjoying a successful season leading into the U.S. Open, as Tumaini Carayol of the Guardian wrote:
A significant shoulder injury has hindered the world's former No. 3 player, who has now made three grand slam semifinals. He was off the tour for a few months to recuperate.
However, he seems well past that injury now, and his win will certainly make a few people happy as Arash Madani of Sportsnet wrote:
Tennis commentator Nick McCarvel summed up the night well:
Dimitrov will now face No. 5 Daniil Medvedev, who beat Stanislas Wawrinka in the quarterfinals. The two men have split their two matches, both of which occurred in 2017. Their rubber match will take place Thursday at a time to be determined.
The winner will advance to the finals and face the last man standing on the opposite side of the draw.
No. 24 Matteo Berrettini will face No. 13 Gael Monfils in one quarterfinal, and No. 20 Diego Schwartzman will look to upset No. 2 Rafael Nadal in the other.
Those matches will take place Wednesday, with the semifinal occurring Friday and the final taking place Sunday.