Serena Williams was sent spinning out of the Western and Southern Open by Maria Sakkari Tuesday after the Greek rallied from a set down to beat the 23-time Grand Slam champion 5-7 7-6 (5) 6-1 and reach the quarter-final.
Williams, 38, raced to a 5-2 in the opening set before Sakkari drew level but the American raised her game again to take the early advantage in the rain-delayed contest.
Williams trailed 3-1 in the second set before reeling off four games in a row and was serving for the match when things began to fall apart.
World No 21 Sakkari dragged the set to a tie-breaker before levelling the contest at 1-1, then surged to a 5-0 lead in the decider and completed the upset win on her eighth match point with a powerful forehand.
Sakkari said she did not play well but was delighted with the victory over the third seed.
Sakkari next faces Briton Johanna Konta, who got past Vera Zvonareva 6-4 6-2.
Earlier, Naomi Osaka put on a solid serving performance to beat Dayana Yastremska 6-3 6-1 and advance to the quarter-final where she will face Anett Kontaveit, who beat Marie Bouzkova 6-3 6-3.
Osaka fired eight aces to zero for Yastremska and needed just over an hour to dispatch the 20-year old Ukrainian.
Former world No 1 Victoria Azarenka improved her record against Alize Cornet to 7-0 with a 6-4 7-5 win to advance to the quarter-final, where the Belarusian will meet Ons Jabeur, who beat Christina McHale 6-3 6-0.
The tournament, the final tune-up before the US Open Grand Slam, is being played at Flushing Meadows in New York instead of Cincinnati due to the Covid-19 pandemic.
