American Sloane Stephens has reached the U.S. Open quarterfinals for the first time by upsetting No. 30 Julia Goerges 6-3, 3-6, 6-1.
Keys next will face the surprise of the tournament, 32-year-old Kaia Kanepi of Estonia, who advanced to the quarterfinals with a 6-4, 6-4 victory over 20-year-old Daria Kasatkina of Russian Federation.
CoCo Vandeweghe made it three Americans into the US Open women's semifinals, beating top-seeded Karolina Pliskova 7-6 (4), 6-3.
"I didn't feel the best", she said in a post-match news conference. "To come together this year, it is a process". She defeated defending champion Angelique Kerber in the fourth round of the Australian Open earlier this season.
Afterwards, Keys ran over to her supporters. They've stuck beside me and helped me through some really low moments.
"I didn't choose to be at this position where I am now, so hopefully I have many more chances".
No stranger to long matches at the US Open, Keys played in the final match on Saturday's schedule and didn't win until 1:45 a.m., and previous year played until 1:48 a.m., the latest finish ever for a US Open women's singles match.
Although Pliskova battled hard to keep her hopes alive on Wednesday, it was Vandeweghe who proved stronger in the key moments of a contest that featured some predictably hard hitting on both sides of the net.More news: Jimmy Kimmel blasts Trump over 'horrible' DACA decision: 'No good reason'
1-6, 6-4, in the late night match at Arthur Ashe Stadium.
Pliskova had a set point at 5-4 but Vandeweghe produced a fine backhand to save it and she was the more consistent player in the tiebreak.
The first came when she seemed down and out in the first set but played a superb tiebreaker.
In the fifth set, as the match stretched beyond 3 1/2 hours, del Potro broke in the last game when Thiem double-faulted. She continued her summer of dynamic tennis serving 72 percent, cranked aces and denied all three break points she faced cruising into a red, white and blue semifinal.
There was finally a break in the decider as Svitolina took advantage of some inconsistent play by the American, who saved a break point with an incredible drop shot but the Ukrainian was still second time lucky, looking poised to close out the match.
"There's no doubt that there's a special sound when the roof is closed", Vandeweghe said. I don't really know. They're spread across all four matches, so technically it's possible we could have an all-American semifinals in NY with a few more upsets.
"I gave my best out there today and I fought, and she was just a bit better than me", Safarova said. We have to admire Serena, Venus, Lindsay [Davenport] and Jennifer [Capriati]. "I didn't feel the best". "Let's make it four for four".
There's plenty of power players left on the women's side, including No. 3 seed Garbine Muguruza facing No. 13 Petra Kvitova in a matchup for former Wimbledon champions.