Daniela Hantuchova became the highest-ranked women's casualty at Wimbledon after losing a three-set marathon to Shinobu Asagoe.
The Slovakian ninth seed went down 6-0 4-6 10-12 in a second-round match that lasted almost three hours.
A routine win had looked on the cards for Hantuchova after she swept her Japanese opponent aside in a one-sided first set.
Asagoe bounced back bravely to level and, in an enthralling final set, both players produced some sparkling tennis before the underdog sealed the upset.
Hantuchova was close to tears after the match and insisted that her fitness was not in question.
"I didn't take my chances. I was very happy in the first set and then suddenly it turned around and she started to play well and saw she had a chance," she said.
"I felt great physically. I just played one of the longest matches ever and felt I could have gone for another two hours but I think the mental side is the one I have to really work hard on.
"It's very frustrating to lose so early here. This tournament
means a lot and I really wanted to do well."
Fourth seed Venus Williams encountered few problems in her second-round match against Slovenia's Katarina Srebotnik, winning 6-4 6-1.
Williams was not at her best but managed to subdue the early fire from Srebotnik to race through the second set.
Srebotnik, ranked 42 in the world, held her own in the opening six games of the match and her hard serving gave the 2000 and 2001 Wimbledon champion problems.
But the match had a predictable feel about it from the moment Williams broke at 3-3 and Srebotnik won just two more games.
Second seed Kim Clijsters joined Zvonareva in the third round with a 6-1 6-3 win over French star Virginie Razzano.
Former champion and sixth seed Lindsay Davenport crushed Rita Grande of Italy 6-3 6-1.
And seventh seed Chanda Rubin also progressed at the expense of fellow American Amy Frazier, winning 6-4 6-4.
But 1th seed Magdalena Maleeva bowed out in three sets to Paola Suarez of Argentina.
The Bulgarian looked to be heading for an easy win when she took the first set, but Suarez stormed back and won the next two to wrap up a 2-6 6-2 6-3 victory.
South African Amanda Coetzer, the 17th seed, crashed out to Italy's Francesca Schiavone, winning the first set 6-4 before losing the next two 7-5 6-4.
Japan's 13th seed Ai Sugiyama is through to the third round, although she had to struggle before beating Eva Fislova of Slovakia 6-1 6-7 (6-8) 7-5.
But Spain's Magui Serna, the 24th seed, is out after losing 6-4 3-6 4-6 to Slovenia's Maja Matevzic.
Russian Vera Zvonareva, seeded 16, progressed at the expense of Spaniard Conchita Martinez Granados, winning 6-3 6-1.