Kim Clijsters exited the only clay tournament of her farewell season at the first hurdle in the J&S Warsaw Cup.
World number four Clijster has 34 career titles
The defending champion and second seed, who is retiring this year, lost 7-6 (7-3) 6-3 to Ukrainian Julia Vakulenko.
World number one Justine Henin enjoyed an easy win over Anna-Lena Groenefeld in her first clay-court match of 2007.
The three-time French Open champion, whose last tournament was over a month ago, eased into the last eight with a 6-1 6-2 over the unseeded German.
Belgian Henin served six double faults but otherwise looked in good form.
"Good periods with my serve and then pretty bad times," said Henin afterwards. "She didn't give me a lot of rhythm so it wasn't easy for me to be into the match."
She hit a lot of aces and good second serves and she kept me on my back foot a little bit
The unseeded Venus Williams also reached the quarter-finals with a 6-1 7-6 (7-4) win over seventh seed Elena Dementieva of Russia.
Fourth seed Jelena Jankovic of Serbia earned a 6-2 7-5 victory over Russia's Elena Vesnina.
Clijsters has reduced her playing schedule ahead of her July wedding to Brian Lynch and only played in Warsaw because she is the defending champion and had promised to return.
"The big difference was the serve," said Clijsters. "She hit a lot of aces and good second serves and she kept me on my back foot a little bit.
"She hit really aggressively and kept me defending all the time.
"A couple of times I tried to step into the return a little bit and be more aggressive, but I felt she was stepping in even more and she went for her shots a lot.
"She was really going for it and hitting a lot of lines and she just played a really good match."