Serena Williams suffered her first loss since 2012 to a rival outside the WTA's top 100, falling to fellow American Shelby Rogers in a Friday quarter-final at the Top Seed Open.
Rogers, ranked 116th, outlasted ninth-ranked Williams 1-6, 6-4, 7-6 (7/5) after two hours and seven minutes, finishing the 23-time Grand Slam champion with a service winner.
"It's every kid's dream growing up to be able to do something like that," Rogers said. "Weird circumstances. Weird surroundings. But a win is a win."
The US Open hardcourt tuneup event at Lexington, Kentucky, is being staged in a quarantine bubble due to the COVID-19 pandemic that had kept Williams idle since a February Fed Cup appearance.
Top seed Williams had not lost to a player outside the top 100 since falling to France's Virginie Razzano in the first round of the 2012 French Open.
Williams, who ousted sister Venus on Thursday, was playing matches to prepare for the US Open, which starts August 31. She will seek a seventh title on the New York hardcourts to match Margaret Court's all-time record of 24 Grand Slam crowns.
"I have only good things to take," Williams said. "I can play a lot better."
Rogers reached her first WTA semi-final since Rio in 2016. She'll face 63rd-ranked Swiss left-hander Jil Teichmann, who ousted American Catherine Bellis 6-2, 6-4.
The other semi-final will send American Jen Brady -- who ousted Czech Marie Bouzkova 6-1, 6-2 -- against US teen Coco Gauff, who won 10 of the last 11 games to rally past Tunisian eighth seed Ons Jabeur 4-6, 6-4, 6-1.
A rain delay of two hours and 45 minutes postponed her match, but Williams showed no sign the wait had put the 38-year-old mother off her game.
Williams broke for a 2-0 lead by forcing an errant Rogers backhand, broke again in the sixth game when Rogers netted a forehand and held on a service winner to claim the opening set after only 26 minutes.
"Definitely a little overwhelmed in the first with that pace," Rogers said of Williams and her powerful serve. "Tried to hit a few more volleys in the second, just hit one more ball."
Rogers, who never faced a break point in the last two sets, broke on her third chance in the last game of the second set when Williams netted a forehand.
Both players held serve into the tie-breaker, where Williams jumped ahead 3-1, only to have Rogers run off four straight points, two on Serena's serve, to seize the lead for good.
Williams netted a backhand to give Rogers two match points, but saved the first with a service winner only to fall when Rogers answered with a service winner of her own.
Rogers, who will return to the WTA top 100 next week, was in her first quarter-final since May 2017, due in part to a long absence after a 2018 left knee injury.