Serena Williams lost in the Top Seed Open quarter-finals after fellow American Shelby Rogers came back from a set down to win their rain-delayed match 6-1 6-4 7-6 in Lexington.
Former world number one Williams dominated the first set but a resilient Rogers forced a decider.
Rogers, ranked 116 in the world, then edged the tie-break to record a memorable victory.
"It's every kid's dream to be able to do something like that," Rogers said.
"Weird circumstances. Weird situation. But a win is a win."
It was Williams' first loss since 2012 to a player outside the WTA's top 100 since Virginie Razzano at 2012 Roland Garros.