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.
The tournament top seed, who came from a set down to beat her sister Venus in the previous round, was playing her first tournament in six months.
Seemingly unaffected by the three-hour rain delay, the 23-time Grand Slam champion looked comfortable in the opening set, showcasing a full variety of shots and big serves, powering the ball from the baseline with some neat footwork.
Williams, 38, could not find a way past Rogers in the second set, who left it late to break her compatriot and take the game in to a third set.
A confident Rogers, 27, played some classy shots of her own in the third set, including a brilliant lob to take the score to 6-5, before a Williams hold set up a tense tie-break.
Rogers, who did not face a break point in the second and third sets, came out on top.