Serena Williams has exited the Western & Southern Open in Cincinnati after a gruelling second-round defeat to Petra Kvitova.
The former world number one raced to victory in the opening round, but despite displaying some of her best tennis of late, she could not replicate the same form to advance to the last 16.
Number eight seed Kvitova applied the pressure from the start, breaking 23-time grand slam winner Williams twice in the first set.
Williams, who has won two titles in Cincinnati before, did not allow the momentum to drop in the second set, breaking her opponent in a marathon third game before grabbing a 5-2 lead and serving out for the set.
A nervy start to the third set saw both players drop their serve, but Kvitova found her stride to go 4-2 up and eventually emerge the 6-3 2-6 6-3 winner after two hours on court.
Meanwhile, number six seed Caroline Garcia fought back from 2-0 down in the first set and 5-3 in the second to overcome Victoria Azarenka 6-4 7-5 in one hour and 36 minutes.
.@Petra_Kvitova scores the win over Williams, 6-3, 2-6, 6-3!— WTA (@WTA) August 15, 2018
Fifth seed Elina Svitolina faced an equally tough match against Svetlana Kuznetsova, with the Russian threatening Svitolina's lead in each set.
With the breaks going back and forth, the Ukrainian eventually held serve to seal a 7-6 (7/1) 4-6 6-4 victory.
Karolina Pliskova, the ninth seed, beat Agnieszka Radwanska of Poland 6-3 6-3, while Belgian Elise Mertens also progressed after her 6-4 6-2 win over Magdalena Rybarikova, as did Australian Ashleigh Barty.
However, 17th seed Naomi Osaka is out after the Japan player lost 6-3 7-6 (10/8) to Maria Sakkari of Greece.
Loved every second of that battle. Always something special to share the court with @serenawilliams.
Thanks @cincytennis fans for staying out late 🙏— Petra Kvitova (@Petra_Kvitova) August 15, 2018
Germany's Tatjana Maria progressed to the second round, where she will take on American world number three Sloane Stephens, the defending US Open champion.
Elsewhere, there were first-round wins for Italian Camila Giorgi and Dutchwoman Kiki Bertens, who defeated American Coco Vandeweghe 6-2 6-0.