Defending champion Sloane Stephens has been knocked out of the US Open in the quarter-finals after defeat by 19th seed Anastasija Sevastova.

The American third seed seemed to struggle physically in high temperatures in New York, with the extreme heat policy again in place.

Stephens hit 27 unforced errors and lost 6-2 6-3 on Arthur Ashe Stadium.

Victory for Latvian Sevastova sees her reach the first Grand Slam semi-final of her career.

She will play the winner of Tuesday’s night match between Serena Williams and Karolina Pliskova in the last four.

The high temperatures have been a significant issue during this year’s championships with the conditions described as “dangerous” in the opening rounds, while Roger Federer said he was “happy the match was over” after his defeat in high humidity on Monday.

“I think it was very physical today and tough to play,” Sevastova said.

“It was so hot in the stadium. I just kept fighting.”

Stephens struggled with her movement, was repeatedly being caught out by Sevastova’s drop shot and after a number of wayward errors was bent double on the court.

She failed to put up a fight in a one-way first set and soon fell 4-1 behind in the second.

Encouraged by her home crowd, the world number three threatened a comeback by getting the match back on serve at 4-3 but was then broken again before Sevastova served out.