Serena Williams was beaten 6-3, 2-6, 6-3 by Petra Kvitova in a thrilling second-round match at the Cincinnati Open on Tuesday, ending the American's bid for a third title at the event.
Williams looked rusty in the first set, struggling to handle the Czech's booming left-handed serve and never created a break-point opportunity.
But the 23-time grand slam champion dug deep in the second, thumping serves and blasting returns to the delight of the supportive crowd under the lights.
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Williams appeared to be in command when she broke Kvitova to start the deciding third set but the two-time Wimbledon champion elevated her service game and was defensively resolute to set up a third-round clash with Kristina Mladenovic.
For former world No 1 Williams, the loss is another setback as she attempts to get back to the top ranking after a difficult childbirth last year.
France's Mladenovic had earlier defeated qualifier Viktoria Kuzmova 6-3, 6-0, while her sixth seeded compatriot Caroline Garcia defeated Victoria Azarenka 6-4, 7-5 to advance to the third round.
Former world No 1 Azarenka, who on Tuesday was given a wildcard for the US Open, never found her stride and committed her fifth double fault on match point to hand Garcia the win.
Big-serving Czech Karolina Pliskova fired an ace on match point to dispatch Agnieszka Radwanska 6-3, 6-3 in their first-round match.
Pliskova had never won a set against Radwanska in seven prior meetings but cruised to the win over the 29-year-old Pole, who is working her way back from injuries.
Greece's Maria Sakkari edged 20-year-old Naomi Osaka in a closely contested second-set tiebreak to dispatch the Japanese 6-3, 7-6 in their first-round match.
Dutchwoman Kiki Bertens cracked eight aces and won 90 per cent of her first serves as she strolled to a 6-2 6-0 win over American Coco Vandeweghe.
Fifth seed Elina Svitolina outlasted Russian Svetlana Kuznetsova 7-6, 4-6, 6-4 in their second-round clash.