Ana Ivanovic became the second high-profile player to tumble out of the Kremlin Cup in two days after losing to Dominika Cibulkova 3-6, 6-2, 7-6 on Wednesday.
The Serbian fourth seed followed Wimbledon champion and sixth seed Venus Williams, who was ousted by Italy's Flavia Pennetta in the first round on Tuesday.
World number five Ivanovic had two match points in the 10th game but the unseeded Slovak fought back to force a tiebreak, where she held her nerve to clinch a memorable victory.
"It was my first match against Ana so I really didn't know what to expect from her," Cibulkova told a news conference.
"In the first set I was nervous and played defensively but in the second and third sets I became more aggressive, putting pressure on her. In the tiebreak, I knew I had to stay aggressive to win the match and that's what I did."
Second seed Dinara Safina avoided the exit after outlasting former world number one Amelie Mauresmo 6-7, 6-4, 6-4 in the last match on centre court which finished near midnight.
Safina led 5-2 in the decider before the Frenchwoman, who has dropped outside the top 20 because of injury and poor form, staged a late rally.
She broke the world number three in the ninth game to get back on serve 5-4 but the Russian hit right back to seal a hard-fought victory.
Olympic champion Elena Dementieva also showed her fighting qualities to outlast Slovenian qualifier Katarina Srebotnik 6-3, 4-6, 7-6 to reach the quarter-finals.
TheRussian third seed hit two successive aces in the tiebreak to end a contest that lasted nearly three hours.
"I'veplayed her several times this season and every time I have had problems with her game," said holder Dementieva who lost to Srebotnik in their last encounter in Tokyo last month.
"Iwas prepared this time, so it paid off."
Fifth seed Svetlana Kuznetsova also reached the last eight, the Russian eliminating Italian Sara Errani 7-6, 6-1,but compatriot Anna Chakvetadze, seeded eighth, disappointed home fans at the Olympic sports complex.
Chakvetadze, who has struggled for form for much of the year, lost to Danish teenager Caroline Wozniacki 2-6, 6-1, 6-4.
Men's third seed Mikhail Youzhny was also shown the door, going down to fellow Russian Teimuraz Gabashvili 7-5, 4-6, 7-6.
World number 22 Youzhny was expected to beat his lesser-known compatriot but paid the price for losing his cool in the tiebreak as he allowed the 66th-rankedGabashvili to reel off the last seven points in the match.
Second-seeded Russian Igor Andreev, who won here in 2005, also bowed out, losing to Frenchman Jeremy Chardy 3-6, 7-5, 6-4.
Russia lost another seed when fifth-seeded Dmitry Tursunov, who won his fifth ATP title in Metz on Sunday, withdrew from his first-roundmatch without hitting a ball with a shoulder injury.
Fourth seed and 2002 winner Paul-Henri Mathieu of France had little problem dispatching Ukrainian Sergiy Stakhovsky 6-3, 6-2.