12:00 AM, September 05, 2018 / LAST MODIFIED: 04:48 AM, September 05, 2018

Stephens sent packing by Sevastova

Anastasija Sevastova of Latvia celebrates after winning the first set against Sloane Stephens of the United States in a quarterfinal at Flushing Meadows on September 4. Photo: Reuters

Anastasija Sevastova ended Sloane Stephens's US Open title defense on Tuesday, toppling the world number three 6-2, 6-3 to reach a first Grand Slam semi-final.

Stephens refused to blame the punishing heat and humidity on Arthur Ashe Stadium, where her inability to convert any of seven break chances in the opening set paved the way for her defeat.

"When you don't play big points well, the match can get away from you," she said.

"I just really couldn't get anything to connect. Even when I did have my break opportunities, I just wasn't playing the points well at all. Mentally, physically, I just wasn't connecting. It just was a really tough day. The heat doesn't make it any more fun."

Sevastova, who stepped away from the game for two years in 2013 after a variety of injuries, will face either 23-time major champion Serena Williams or Karolina Pliskova for a place in the final.

With the departure of third-seeded Stephens, eighth-seeded Czech Pliskova is the only top-10 woman left in the draw.

Six-time US Open winner Williams is seeded 17th as she battles for her first Grand Slam title since the birth of daughter Olympia during last year's tournament.

The US superstar, who can match Australian Margaret Court's all-time record of 24 major titles with a victory, fired an impressive 18 aces in a three-set victory over hard-hitting Estonian Kaia Kanepi in the fourth round.

She'll likely need to be at full speed on her serve against Pliskova, the three-time WTA tour ace leader who briefly ascended to world number one last year.

She and Pliskova have history at Flushing Meadows.

The eighth-seeded Czech toppled Williams to reach her first Grand Slam final here in 2016.

"I know she has a big game, but I have a big game too," Pliskova said.

Sevastova said she'd watch Tuesday night's match with interest to see who emerges as her last-four foe.

"It's going to be a close match, I think," she said.

Sevastova herself relies on variety more than power, and she caught a slow-moving Stephens with a number of drop shots and drop-shot lob combinations.

After taking a 4-1 lead in the second set, Sevastova admitted that memories of last year's quarter-final against Stephens flashed across her mind.

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

Indeed Stephens fought back to narrow the gap to 4-3, but Sevastova grabbed another break with a well-timed drop shot for a 5-3 lead and sealed the victory on her third match point when a weary Sephens put a backhand into the net.

"I lost my nerves a little bit," Sevastova admitted. "I think she lost also her nerves a little bit, it's normal. It's for semi-finals of US Open."