American Frances Tiafoe reached the U.S. Open semifinal for the second time in three years on Tuesday after Bulgarian Grigor Dimitrov from their match with a leg injury while trailing 6-3 6-7(5) 6-3 4-1.

Tiafoe with next meet 12th seed Taylor Fritz in an all-American clash with both players aiming to break a 21-year U.S. men's major drought in a tournament where many of the top contenders were eliminated early on.

Ninth seed Dimitrov, who had to retire with a groin injury during his fourth round tie against Daniil Medvedev at Wimbledon this year, left after the third set for medical treatment and slogged through a few games in pain as his team urged him to retire.

Tiafoe had a muted celebration and offered his opponent an embrace at the net, with the crowd already starting to shuffle out of Arthur Ashe Stadium.

"It was a really, really high-level match," said Tiafoe. "Obviously I didn't want to end it like that."

Tiafoe was in superb form as he broke his opponent with a perfectly placed backhand winner in the fifth game and got another break on the fourth set point when Dimitrov whacked a forehand into the net.

It looked as though Tiafoe might run away with it as he went up 4-1 in the second set but Dimitrov broke back in the seventh game as the American’s unforced errors began to pile up.

Dimitrov won four points in a row to go up 6-3 in the tiebreak but nearly let it slip through his fingers with a pair of double faults before Tiafoe handed it to him on set point with a double fault of his own.

Down a break in the third set, Dimitrov winced after stepping awkwardly along the baseline in the final game and a double fault gifted Tiafoe the set for a 2-1 lead.

Dimitrov limped off court to receive treatment from the physio before the fourth set and was shaking his head as he moved awkwardly around the court in the first game of the final frame.

Tiafoe, the 20th seed, faces the biggest match of his life career against Fritz and will look to get the better of another American after taking out Ben Shelton in a five-set, third-round thriller.

"I can't be more excited," said Tiafoe. "I know we're two Americans but I hope you're with me come Friday.