Madrid - Russian Nadia Petrova brought Serena Williams's hopes of improving her Madrid tournament credentials to an abrupt end on Wednesday by beating the world number one in the third round.
Top seed Williams, who pulled out of the clay court tournament in the first round on her debut last year, got off to a positive start before seeing the match go in Petrova's favour for a 4-6 6-2 6-3 victory.
Older sister Venus will be confirmed as the number two player in women's tennis behind Serena when the WTA rankings are next published Monday, after she reached the Madrid quarter-finals on Wednesday.
However Serena's failure to get past the third round here will give her rivals hope only 10 days before the start of the French Open at Roland Garros.
Having only narrowly beaten another Russian, Vera Dushevina, on Monday, Williams looked to be heading to certain victory after world number 18 Petrova had to have a long massage after losing the first set.
However the Russian re-emerged to take control of the next set and, as Williams failed to produce a challenge, the third set and the match.
Number 16 seed Petrova, who beat Williams in their last encounter, at Beijing in September 2009, will now meet Czech Lucie Safarova or Romanian Alexandra Dulgheru in the quarter-finals.
Singles Results (x denotes seeding):
Marin Cilic (CRO x8) bt Edoardo Schwank (ARG) 6-3, 6-0
Victor Hanescu (ROM) bt Daniel Munoz De La Nava (ESP) 6-0, 4-6, 6-3
Gael Monfils (FRA x12) bt Philipp Petzschner (GER) 1-1, retired - leg injury
Juan Monaco (ARG) bt Thomaz Bellucci (BRA x16) 6-2, 6-2
Feliciano Lopez (ESP) bt Oscar Hernandez (ESP) 6-1, 6-2
Ernests Gulbis (LAT) bt Mikhail Youzhny (RUS x10) 7-6 (7/2), 6-4
Rafael Nadal (ESP x2) bt Oleksandr Dolgopolov (UKR) 6-4, 6-3
John Isner (USA x13) bt Santiago Giraldo (COL) 1-6, 7-6 (8/6), 6-2
Stanislas Wawrinka (SUI x15) bt Leonardo Mayer (ARG) 6-4, 4-2, retired
Nicolas Almagro (ESP) bt Robin Soderling (SWE x4) 6-4, 7-5
Jelena Jankovic (SRB x7) bt Ana Ivanovic (SRB) 4-6, 6-4, 6-1
Arantxa Parra Santonja (ESP) bt Peng Shuai (CHN) 1-6, 7-6 (7/1), 6-3
Venus Williams (USA x4) bt Francesca Schiavone (ITA x15) 3-6, 6-1, 6-2
Nadia Petrova (RUS x16) bt Serena Williams (USA x1) 4-6, 6-2, 6-3