“That was a war, epic match, so physical,” Shelton said. “Carlos is one of the greatest champions in our sport already at 23 years old.

“It’s always fun being on the court against him. It’s ridiculous some of the stuff he comes up with sometimes – the most enjoyable match that I’ve had in my career, for sure.”

Alcaraz had gone from strength to strength through his first four matches, but Shelton posed his biggest test and finally proved too much.

Shelton, pumped up by the Arthur Ashe Stadium crowd, reeled off nine straight games bridging the second and third sets to seize control, capitalising on Alcaraz’s flagging serve and getting the better of him in rallies that left him winded.

Alcaraz stopped the rot, mustering a break chance as Shelton served for the third set before the American closed it out.

It was a sign Alcaraz was far from a spent force, and he roared back to take the fourth set in 36 minutes.

The margins in the fifth set were razor thin. Alcaraz had a break chance at 2-2, then it was Shelton’s turn at 4-3, but the match was destined to go all the way, to a 10-point match tiebreak that Shelton capped with an ace on match point.

After the first three matches of the day all went the distance, Alcaraz and Shelton didn’t get underway until after 11pm – and both exploded out of the gate.

Neither man faced a break point through the first 10 games, although Alcaraz came under some self-inflicted pressure with back-to-back double faults that made for a complicated hold for 4-4.

Alcaraz put together his first love hold for 5-5 then grabbed the first break in the next game, forcing a forehand error from Shelton as the American was feeling the pressure at 0-40 down.

That left the Spaniard to serve for the set, but two errors to start the game were all the opening Shelton needed.

He broke to force the tiebreaker and seized a 5-3 lead only for Alcaraz to win the last four points and pocket the set after 61 minutes.

But Alcaraz couldn’t maintain the pace and as his struggles on serve increased, Shelton roared away, two of his three service breaks coming courtesy of double faults from the Spaniard, including one on set point.

“I knew that my serve was only going to get better, and if I had already dialled in the return to the point that I could break him, I thought my chances were good to get that second set,” Shelton said. “Once I got rolling the second set, broke him three times in that set, it was a lot easier to just play and swing more freely.”

Earlier, Tiafoe rallied from two sets down – and 3-0 down in the fifth set – to hand fellow American Alex Michelsen a defeat.

Tiafoe edged 22-year-old Michelsen 5-7, 3-6, 7-5, 6-3, 7-6 (10/6) to reach the semi-finals at Flushing Meadows for the third time in five years.

American 11th seed Tiafoe had all he could handle from his young compatriot, who hadn’t dropped a set in reaching the first Grand Slam quarter-final of his career.

He was on the brink of another three-set win when he served for the match at 5-4 in the third, but a pair of double faults opened the door to Tiafoe and the more experienced player ploughed through.

“I feel like I just handed it to him,” a dejected Michelsen, ranked 46th in the world, said. “Then after that he started playing really good.”

It took two more sets before Tiafoe landed a forehand winner to clinch the 10-point match tiebreaker after four hours and 38 minutes, Michelsen weeping in the winner’s arms as they met at the net.

“I was trying to keep it together, and I didn’t do a good job of that,” Michelsen said. “The match was very emotionally draining. Frances, he’s such a good guy and he said a lot of personal things and just kind of got to me.”

Michelsen had come out swinging on Arthur Ashe Stadium against an uncharacteristically passive Tiafoe, who usually feeds on the energy of the crowd.

“They were flat, but I was even more flat than the fans,” Tiafoe said. “I was joking about that, but they definitely woke up as I started to pick it up.”

After surviving the third set, he had a new lease of life, and even when Michelsen responded with an early break in the final set Tiafoe refused to fold.

“The moment is huge,” said Tiafoe, who next faces good friend Shelton.

“I’ve won a lot of great matches here. I’ve lost a lot of tough matches here as well. It feels amazing to get this done.”