Sloane Stephens is out of the US Open, losing 6-2 6-3 to Latvian Anastasija Sevastova in the quarterfinals.

The reigning champion, who beat Sevastova in the same round last year en route to her first Grand Slam title, missed numerous chances to grab an early lead in the rematch and could never get back into the match.

Read more: Roger Federer crashes out of US Open

Sevastova, the No. 19 seed, will now face Serena Williams in her first Grand Slam semifinal. Williams fought back from a slump in her first set to go on and defeat Czech tennis star Karolina Pliskova with a stronger performance in the second set.

Three-quarters of Arthur Ashe Stadium was in the sun on another day of more than 90-degree temperatures in New York, and Stephens seemed to lack some of her usual extensive court coverage in the heat.

"Mentally, physically, I just wasn't connecting," Stephens said. "It just was a really tough day. The heat doesn't make it any more fun."

Stephens said she had been battling a cold, but her biggest problem was her serve. The third seed was broken five times in the 84-minute match and squandered all seven break-point chances in the first set.

Stephens, who was visibly struggling in the conditions, simply had no answer for the steady stream of drop shots sent her way by Sevastova

With the match getting away from the American, her frustration became apparent as she started gesturing to her coach and screaming out. "I'm trying!" she responded to a plea from the crowd to pick it up in the second set.

For Sevastova, who retired in May 2013 with her body battered by muscular and back-related injuries, her success story goes on. She returned nearly two years later and finally broke through on her third straight appearance in the US Open last eight.

"It was an amazing journey, this three, four years," she said.

mds (AP, Reuters)