NEW YORK (AFP) - Sloane Stephens' US Open title defence ended with a 6-2, 6-3 quarter-final defeat by Anastasija Sevastova on Tuesday (Sept 4) as the Latvian avenged a 2017 loss to the American at the same stage.

Sevastova, through to the semi-finals of a Grand Slam for the first time, will face 23-time Major singles champion Serena Williams - who beat eighth-seeded Czech Karolina Pliskova - for a place in the final after toppling Stephens in brutal heat and humidity on Arthur Ashe Stadium.

"I think it was very physical today. It's tough to play, so hot in here. The first set was so important," said the 19th-seeded Sevastova, who broke Stephens in the final game to pocket the opening set.

She earned the last of five breaks in the second set to take a 5-3 lead and served it out on her third match point when a weary Stephens put a backhand into the net.

"I showed some nerves at the end but it's normal," Sevastova said.

With the heat, humidity and blazing sun combining to make conditions hazardous enough that organizers suspended junior matches, the scraps of shade on the massive Ashe court were invaluable.

"When the shade was there it was so hot here and amazing there," Sevastova said of the contrast of playing on the shady or sunny end of the court.

"The last games, it was good that I played in the shade. The sun was not comfortable and it was hot."

Stephens was ranked 83rd in the world when she upset Sevastova in three sets in the quarter-finals last year, trailing by a break in the third set before winning it in a tiebreaker.

Now number three in the world, Stephens joined an exodus of women's seeds that saw top-ranked Simona Halep and number two Caroline Wozniacki exit in the first two rounds.