WARSAW, Poland -- Qualifier Julia Vakulenko beat second-seeded Kim Clijsters 7-6 (3), 6-3 to advance to the quarterfinals of the clay court J&S Cup on Thursday.
Vakulenko, a Ukrainian ranked 61st in the world, hit seven aces to avenge a second-round loss to Clijsters in last year's J&S Cup, which Clijsters eventually won.
"I really enjoy playing against top players," Vakulenko said.
In other matches, Venus Williams of the United States beat seventh-seeded Elena Dementieva of Russia 6-1, 7-6 (4); top-seeded Justine Henin of Belgium downed Anna-Lena Groenefeld of Germany 6-1, 6-2; and fourth-seeded Jelena Jankovic of Serbia defeated Elena Vesnina of Russia 6-2, 7-5.
Clijsters struggled to find her range from the baseline throughout the match.
At 5-3 in the tiebreaker, the former top-ranked player sent a forehand wide, then netted another forehand to give Vakulenko the first set.
The Belgian's troubles continued in the second set, where she netted consecutive groundstrokes to hand Vakulenko a 4-2 lead and the only break she would need.
"She hit a lot of aces, a good second serve and kind of had me on my back foot a little bit and always at a little bit of a disadvantage," Clijsters said. "She just played a really good match and was better than me."
Williams grabbed control of the first set when Dementieva hit a forehand long to put Williams ahead 2-0.
"In the beginning it was very tough to get into the game because she was so powerful, very aggressive," Dementieva said. "It just took me a few games to get into the rhythm of the game and be able to have a long rally."
Both players struggled with their serves in the second set. Tied 2-2, neither held serve for the rest of the match.
At 5-4 in the tiebreaker, Williams hit a forehand winner and then Dementieva pushed a two-handed backhand long to end the match.
Henin, playing for the first time since losing to Serena Williams in three sets at the Sony Ericsson Open last month in Miami, had little trouble against the 21-year-old Groenefeld.
"I just tried to stay very focused and do my job and finally won pretty easily and that's positive," Henin said.