Angelique Kerber suffered her first defeat as world number one when Petra Kvitova beat her 6-7(10), 7-5, 6-4 in a marathon third- round match at the Wuhan Open in China on Wednesday.
German top seed Kerber saved three set points before a forehand error from Czech Kvitova handed the US Open champion a tense first-set tiebreak.
But Kvitova, showing the battling qualities that have brought her two Wimbledon titles, fought back to force a decider.
Kvitova, seeded 14th, broke Kerber early in the third set to lead 4-2 and despite struggling with cramp, she held her nerve to convert her seventh match point after three hours and 20 minutes on court.
"At the end I think there is no loser. I did everything today. I was fighting until the end," Kerber told reporters.
Kvitova will face Britain's Johanna Konta in the quarter-finals.
