Frances Tiafoe pulled off the biggest comeback of his career Tuesday at the US Open, rallying from two sets down to stun Alex Michelsen in a five-set marathon and reach the semifinals at Arthur Ashe Stadium for the third time in five years.

Tiafoe won 5-7, 3-6, 7-5, 6-3, 7-6(6), erasing a deficit that looked all but insurmountable against a red-hot Michelsen, who had arrived at the quarterfinal as the only man left in the draw without having dropped a single set. Michelsen held a commanding lead through two sets, and Tiafoe entered the match with a career record of just 2-17 at Grand Slams when trailing two sets to love, according to CBS Sports' recap of the match.

From 40% First Serves to a Tiebreak for the Ages

Tiafoe struggled badly with his serve early, posting a first-serve percentage below 40% through the opening three sets, but found his range as the match wore on and forced a decisive fifth set. After falling behind a break in the fifth, he clawed back to force a tiebreak — and closed it out 10-6 in the extended breaker required at that stage of the match.

THE FIRST COMEBACK FROM TWO SETS DOWN IN A US OPEN MEN'S QUARTERFINAL SINCE ROGER FEDERER DID IT IN 2014

That's how ESPN described the scale of Tiafoe's win, drawing a direct line to Federer's comeback against Gael Monfils more than a decade ago. Michelsen, who fell to 0-3 for his career in five-set matches, embraced Tiafoe at the net afterward visibly emotional following his first loss of the tournament.

The win sends the 27-year-old American into his third US Open semifinal in the last five years, with a maiden Grand Slam final still eluding him. He'll face the winner of Wednesday's quarterfinal between second seed Carlos Alcaraz and eighth seed Ben Shelton, guaranteeing at least one American in Friday's semifinal on the men's side alongside Tiafoe.

The result electrified the Ashe Stadium crowd, which has increasingly rallied behind Tiafoe as a hometown favorite in recent US Open runs, and instantly became one of the most replayed matches of this year's tournament.

What's next: Tiafoe's semifinal matchup will be set once Alcaraz and Shelton finish their quarterfinal Wednesday, with the men's semifinals scheduled for Friday at Arthur Ashe Stadium.