The Swiss world No. 3 fell in five sets on Tuesday.
No. 3 Roger Federer suffered a stunning upset loss to No. 78 Grigor Dimitrov in five sets (6-3, 4-6, 6-3, 4-6, 6-2) in the U.S. Open quarterfinals on Tuesday, ending his hopes of winning his 21st Grand Slam title.
Federer broke Dimitrov in his first return game en route to a 3-0 lead in the first set before holding on to take it, 6-3. Dimitrov bounced back in the second set, breaking in the sixth game to go up 4-2. He then served for the second set at 5-3 but dropped his serve to Federer, whose wide forehand shot gave Dimitrov the 6-4 win.
Federer appeared to regain some momentum in the third set, taking advantage of Dimitrov's double-fault on a break point at 3-3 and winning the next three games. Federer was plagued by 19 unforced errors, 11 on the forehand side, in the fourth set, and the five-time champion could not convert five break points when Dimitrov served for the win.
After Federer called for a medical timeout between sets, Dimitrov jumped to an early 4-0 lead in the deciding fifth set thanks to a double break before the 38-year-old Swiss got on the board. Dimitrov clinched the set three games later.
Federer's loss follows No. 1 Novak Djokovic's and No. 1 Naomi Osaka's, who exited the tournament on Monday.
Before Tuesday's victory, Dimitrov had never beaten Federer in seven previous meetings. He is now the lowest-ranked Grand Slam semi-finalist since Rainer Schuettler did so as world No. 94 at Wimbledon 11 years ago.
Dimitrov will face No. 5 Daniil Medvedev in the semifinal on Friday at 11 a.m. ET.