US Open defending champion Sloane Stephens sent packing by Anastasija Sevastova

Defending US Open champion Sloane Stephens was dumped out in the quarter-finals after losing 2-6, 3-6 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 semis, 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 US 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.

Serena Williams fought back from a break down in the first set to beat Czech eighth seed Karolina Pliskova 6-4 6-3 and reach the semi-finals.

The 36-year-old, who is bidding for a record seventh title at Flushing Meadows as well her 24th career Grand Slam crown, started sluggishly at the Arthur Ashe Stadium , allowing Pliskova to take a 3-1 lead in the opening set.

With the heat rule in effect and junior matches on the outside courts suspended earlier in the day, Williams made 22 unforced errors in the opening set but showed all of her big-match pedigree to battle back and close out the set by winning the last four games.

“The crowd was really rooting for me and I felt so bad because everyone out here was cheering and I wasn't winning so I thought I had to try harder,” Williams said courtside.

“I really feel like right now I'm playing free. I was having a baby this time last year so I have nothing to prove.”

The second set was far more comfortable for the number 17 seed.

Two successive breaks of serve allowed Williams to win her eighth game in a row and open up a 4-0 lead, and she saw off a brief fightback from Pliskova before sending down her 13th ace to seal the match in one hour and 26 minutes.

“When I get to a Grand Slam, I feel I want to play really hard. I want to do really well. I love this game so maybe that's why,” added Williams, who was beaten by Pliskova in the 2016 semi-finals.