China's Li Na recovered from a terrible start to pull off a scarcely credible 0-6 6-1 6-4 victory over world number one Serena Williams in the second round of the Stuttgart Grand Prix on Thursday.
Williams, playing her first match at the indoor tournament after receiving a first-round bye, took advantage of a series of errors from her unseeded opponent to take the first set in just 21 minutes.
It looked a complete mismatch at that stage but in the second set Li suddenly found her range, hitting the lines more and more often and putting Williams under increasing pressure.
Williams, who won her ninth grand slam title at the US Open last month, made 11 unforced errors in the second, dropping her serve twice along the way, and despite improving in the third she was unable to re-establish herself.
Li broke for 4-3 in the decider and while Williams saved three match-points in her next service game she was unable to stop her opponent serving out for the match, which she clinched with an ace down the middle.
Li was joined in the quarter-finals by Victoria Azarenka of Belarus, who upset eighth-seeded Pole Agnieszka Radwanska 6-1 7-5.
Patty Schnyder of Switzerland earlier overcame Russian fifth seed Svetlana Kuznetsova 6-4 4-6 7-5 in an entertaining first round match.
Serving strongly throughout, the 29-year-old Schnyder set up match-point when Kuznetsova marginally overhit a return and clinched it when the Russian's attempted backhand winner drifted wide.
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