NEW YORK, NY, USA -- No.19 seed Anastasija Sevastova of Latvia ended the title defense of No.3 seed Sloane Stephens of the United States, with a stunning 6-2, 6-3 upset in the quarterfinals of the 2018 US Open on a sweltering Tuesday afternoon.

"I think it was third time lucky," said Sevastova, who had reached the US Open quarterfinals in each of the last three years, during her press conference. "But again, if I would lose I would also be proud of myself. Three times quarters. It's not happening every year or every day."

Read more: As it happened: Flawless Sevastova keeps her cool to make US Open history

25-year-old Stephens squeaked past Sevastova in the same round last year en route to her maiden Grand Slam title, but this year, it was Sevastova who prevailed in a scant 83 minutes, becoming the first player representing Latvia to reach the final four at the US Open.

"Tough loss last year, but every loss you learn something, and I was okay if we would play three sets," Sevastova stated. "I was also thinking about that match at 4-1 in the second set, but it happened. I was still proud of this match. I think it was a great match last year, and look what happened now?"

28-year-old Sevastova made the first Grand Slam semifinal of her career by hitting 18 winners to 13 for Stephens, while firing only one more unforced error than the American. Most importantly, Sevastova converted five of eight break points, while saving seven of the nine against her, in the extremely hot conditions.

"When you don't play big points well, the match can get away from you," Stephens said during her post-match reflections to the media. "I think that's what happened today. I didn't convert. I didn't play the big points well, and you don't win matches when you don't take your opportunities."

Sevastova fared better in the heat on the day, and the Latvian said the conditions are "always tough in the beginning, I think, but then you get in the flow and you just forget it. I think shade was also pretty tough. It's moving, and, yeah, one side with the sun was very hot. Very hot."

"It's always tough to play [Stephens], I think, because her ball is so heavy," Sevastova continued. "She has big spin on the forehand. Also backhand. It's like it comes at you so fast. Also good serve and moving well. It's tough to make points against her, but I had to be aggressive. I had to try to come to the net, play some dropshots, move her, not let her play the game with her forehand, and make the winners."

In the semifinals, Sevastova will face the winner of the Tuesday night quarterfinal between two former World No.1 players, six-time US Open champion Serena Williams of the United States or 2016 US Open finalist Karolina Pliskova of the Czech Republic.

"I hope I will enjoy it, playing semifinals," Sevastova said. "It's not every day you play semifinals of a Grand Slam. Either way, against Serena or Karolina, it's going to be a tough match. They play aggressive. They serve big. I need to find some solutions. Some dropshots," she concluded, with a smile.

Stephens came out attacking, blasting through rallies with compact, effective groundstrokes from both sides, giving herself four chances to break at 1-1. But Sevastova methodically picked off each of those chances, and ended up with a crucial hold in a long game for 2-1.

Sevastova pounced after that escape, quickly breaking an error-prone Stephens for a 3-1 lead, changing the dynamic of the match. Stephens again had opportunities in the next game, holding three break points, but the Latvian used her stellar backhand and her wily lob to deny those as well, and she held for 4-1, saving seven break points in her last two service games.

That gave Sevastova all the momentum she needed for the rest of the opening set, as she used her highly regarded drop shot to hold for 5-2, and she dismantled Stephens in the American’s last service game, breaking at love behind consecutive backhand winners for the one-set lead.

Sevastova opened up an even wider lead as the second set commenced, using a stunning backhand passing winner to hold for 1-0, and continued to frustrate Stephens into errors as she broke the American for a 2-0 lead.

Stephens twice pulled back on serve at 2-1 and again at 4-3, but both times, the American’s euphoria was short-lived, as Sevastova would reclaim her break lead immediately. At 4-3, in particular, Stephens had four game points to hold for 4-4, but Sevastova prevailed, closing out the game with two perfect backhand drop shots, thereby reaching 5-3.

"I lost my nerves a little bit," Sevastova said of the up-and-down second set. "I think she lost also her nerves a little bit. It's normal, I think. It's for semifinals of US Open. It's okay, I think, just to show some emotions."

Serving for the match at 5-3, the Latvian zipped to triple match point, and though a determined Stephens saved two, the American punched a backhand into the net on the third, and an emotional Sevastova was bound for the first Grand Slam semifinal of her career.