Felix Auger-Aliassime moved into the US Open semi-finals when Spanish teenager Carlos Alcaraz retired due to an abductor injury while trailing 6-3 3-1.

The 18-year-old Spaniard showed no signs of distress as Auger-Aliassime took the opening set but after the Canadian broke to open the second Alcaraz slumped.

During the first changeover of the second set, Alcaraz called for the physiotherapist but did not receive any treatment. A game later, however, the Spaniard turned and indicated he could not go on.

"It's really tough to end a great tournament like this," said Alcaraz. "I had no choice to still playing.

"First of all I have to take care of my body.

"I didn't feel good to still playing, so I had to retire."

Coming off gruelling back-to-back five-set matches, including a third-round win over Stefanos Tsitsipas, Alcaraz stepped onto the court with both legs heavily strapped and seemed to lack the power and energy that had carried him to the quarter-finals.

"I didn't see it coming. It's unfortunate to finish like this," said Auger-Aliassime. "I didn't really see anything.

"Either I was too focused on my game or I'm not seeing things."

Auger-Aliassime becomes the first Canadian man to reach the US Open semi-finals where he will take on second seeded Daniil Medvedev of Russia.

Medvedev ended the run of qualifier Botic van de Zandschulp to reach the semi-finals of the US Open for a third straight year.

Van de Zandschulp became one of the most unexpected slam quarter-finalists in recent memory with a five-set victory over Diego Schwartzman in the previous round, and he dug in admirably to become the first player all tournament to take a set off Medvedev.

But by then the 25-year-old Dutchman, who had never even been to New York prior to arriving for qualifying last month, was two sets down and it was the second seed who came through 6-3 6-0 4-6 7-5.

Medvedev had romped through his first four matches, spending less than eight hours on court compared to more than 13 for Van de Zandschulp, who was looking to become the first qualifier to reach the men's semi-finals in New York.

And it was a mismatch for most of the first two sets, with the only high point for the qualifier coming when he battled back from 1-5 to 3-5 in the opening set.

But he was not prepared to let his big moment drift away and, with a slight drop in level from Medvedev, Van de Zandschulp broke to lead 3-2 in the third set and, cheered on by the crowd, held his advantage to clinch the set.

A decider proved beyond him, though, as pressure from Medvedev finally told with a break in the final game.

The Russian has a superb record on hard courts and he made his first grand slam final in New York two years ago, losing in five sets to Rafael Nadal, before losing a tight contest against eventual champion Dominic Thiem in the last four 12 months ago.

His sights will be firmly on the trophy, while Van de Zandschulp can reflect on a superb fortnight that will see his ranking climb from 117 to just outside the top 60.