US Open Tennis 2018 Results: Scores, Highlights from Early Tuesday Results

There will be a fresh winner at the 2018 U.S. Open in New York after defending champion Sloane Stephens was shocked by Anastasija Sevastova in straight sets on Tuesday.

The Latvian outclassed the American, as the exceptional heat appeared to affect the reigning queen of Flushing Meadows in the quarter-finals.

Sevastova conceded only five games on her way to victory, winning 6-2, 6-3, in a comprehensive display as the No. 3 seed exited.

Juan Martin del Potro overcame the challenge of John Isner, with the No. 3 seed coming from behind to seal the win.

Isner captured the opener, but Del Potro battled hard to win three sets in a row to move into the last four.

Both men were clearly fatigued at the end of the match, as the Argentinian hauled himself over the line, 6-7 (5), 6-3, 7-6 (4), 6-2.

Here's a selection of early results and evening schedule for Tuesday's quarter-finals:

Arthur Ashe Stadium

(19) Anastasija Sevastova def. (3) Sloane Stephens 6-2, 6-3

(3) Juan Martin del Potro def. (11) John Isner 6-7 (5), 6-3, 7-6 (4), 6-2

Evening Session (begins 7 p.m. ET, 12 p.m. BST Wednesday)

(17) Serena Williams vs. (8) Karolina Pliskova 

(1) Rafael Nadal vs. (9) Dominic Thiem

Tuesday Recap

Andres Kudacki/Associated Press

Stephens had looked good as fellow seeds crashed out in the Big Apple, but the 25-year-old ran out of gas before she could make the semi-finals.

The Florida-born champion was in trouble from the earliest moments as she failed to settle, and Sevastova was defiant as she quickly built a lead.

The unforced errors quickly added for Stephens, and her opponent claimed the opening set 6-2.

Stephens appeared drained and dejected in her chair, but she mustered energy for a small comeback in the second.

Alex Pantling/Getty Images

The No. 3 seed had struggled with Sevastova's variation of shot, and the 28-year-old was dominant as she passed her opponent multiple times as she attacked the net.

The tactics were not working for the American, but she rallied to 4-3 in the second before losing form once again.

The Latvian completed the victory, 6-2, 6-3, and she will now meet either Williams or Pliskova in the last four.

The U.S. Open highlighted the winner's journey on Twitter:

Isner showed why he has improved his ranking in recent years, but Del Potro had too much quality when it really mattered.

The No. 11 seed edged the first set in a dramatic tiebreak, but Del Potro was in better form during the second set.

The Argentinian claimed the second 6-3, but Isner stayed with his opponent in the third. However, he was disappointed as Del Potro edged further ahead with a tiebreak success, as both men gave huge entertainment to the crowd in the heat of the afternoon.

Elsa/Getty Images

Del Potro received treatment in the final set as he remained stationary in his seat, but Isner could not take advantage after draining his own supplies.

The 29-year-old took the win as he closed out the set 6-2, but Isner was given a standing ovation by his home crowd.