Defending US Open champion Sloane Stephens was dumped out in the quarter-finals after losing 6-2 6-3 to Latvian 19th seed Anastasija Sevastova in sweltering conditions on Tuesday.

The third-seeded American lacked the swagger she showed through the opening four rounds and wilted on the sun-baked Arthur Ashe Stadium court against Sevastova, who became the first Latvian woman to reach the semi-finals of the US Open.

The 28-year-old Sevastova, who avenged last year's quarter-final loss to Stephens in New York to reach her first Grand Slam semi, converted five of her eight break points and saved seven of the nine against her.

"I think it was very physical today," Sevastova, who lost to Stephens during a U.S. Open tune-up in Montreal last month, said during her on-court interview. "It was tough to play."

With temperatures hovering above 90 degrees Fahrenheit (32.2°C), and the tournament's extreme heat policy in effect for a second consecutive day, Sevastova broke an error-prone Stephens to go ahead 3-1 and never looked back.

Sevastova, who retired from tennis in 2013 due to injuries before returning in 2015, withstood a fightback from the 25-year-old American late in the match but sealed the win on her third match point when Stephens sent a backhand into the net.

Next up for Sevastova is the winner of the evening match between six-times champion Serena Williams and Czech eighth seed Karolina Pliskova.