NEW YORK: Pablo Carreno Busta outlasted an erratic Denis Shapovalov 3-6 7-6(5) 7-6(4) 0-6 6-3 on Tuesday in a seesaw battle lasting over four hours to advance to his second US Open semi-final.

The 29-year-old Spaniard, the oldest of the men's quarter-finalists at Flushing Meadows, looked to be heading for his first loss on hardcourts to his Canadian opponent after Shapovalov served him a bagel in the fourth set.

However, after getting some treatment to his lower back, 2017 semi-finalist Carreno Busta fought back gamely to seal the deciding set in a match that went past 1 a.m. in New York.

After spending just an hour on court in his previous round when world number one Novak Djokovic was disqualified, Carreno Busta was understandably drained.

"I'm destroyed, but you know I'm very, very happy," he said in his on-court interview. "After this fight, this battle, it's hard to say, but it is incredible to be back in the semi-finals again."

Carreno Busta had won three of his previous four meetings with Shapovalov, including a win over the Canadian at Flushing Meadows in 2017 when the Spaniard made the semi-finals.

The 20th seed had not lost a set to Shapovalov in his three hardcourt wins but that changed quickly on Tuesday as the 21-year-old broke his serve a second time before taking the opener.

Shapovalov's serve began to falter, though, as a dogged Carreno Busta rallied to take the next two sets via tiebreakers.

The Canadian hit 26 aces but also committed 11 double faults -- many of them at crucial moments.

Shapovalov, who has now lost in five sets in his last three visits to Flushing Meadows, struck 76 winners but it was the same number of unforced errors that proved his undoing.

The crucial break in the decider came in the sixth game for Carreno Busta, who could convert only five of his 21 breakpoint opportunities, and he sealed the contest on his first match point when Shapovalov found the net on his return.

"I think I'm very comfortable on these courts," said Carreno Busta. "I think the last few months during quarantine were very, very tough for everybody. But I worked very hard with my coach. When you work hard, normally the results arrive.

"Maybe not always in the first tournament or on the second, and I was a bit lucky and now I am in the semi-final."

Carreno Busta, who will be eyeing a place in his maiden Grand Slam final, will face Alexander Zverev in the last four after the German took down Borna Coric earlier on Tuesday.