Clijsters paid the price for a reduced playing schedule ahead of her July wedding to Brian Lynch in a poor second round performance against the Ukrainian qualifier.
After playing no matches in nearly a month, Clijsters has scheduled no other clay court events this year and played in Warsaw only because she is the defending champion and had promised to return.
But the number two seed was never able to get the upper hand against an enterprising opponent that she beat easily at the same stage a year ago.
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"The big difference was the serve," Clijsters said. "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 added.
"She was really going for it and hitting a lot of lines and she just played a really good match."
Fellow-Belgian Justine Henin, the top seed, moved into the quarter-finals with a 6-1 6-2 victory over Germany's Anna-Lena Groenefeld.
Fourth seeded Serb Jelena Jankovic and unseeded American Venus Williams, who upset seventh seed Elena Dementieva of Russia , also went through.
Henin was playing her first match since reaching the Miami final over four weeks ago, but she was never troubled by Groenefeld, who has only two wins from seven tournaments this year.
It took the German 20 minutes to earn her first game point, but she competed better in the second set when most games went to deuce.
Although Henin's acclaimed backhand was working well, six double-faults marred what was otherwise a fine performance.
"Good periods with my serve and then pretty bad times," said Henin. "She didn't give me a lot of rhythm so it wasn't easy for me to be into the match. I just tried to do my job the best I could and to stay very focused."
Jankovic earned a 6-2 7-5 victory over Russia's Elena Vesnina without having to raise her game much above the ordinary, while Williams had to work hard to close out a 6-1 7-6 win over Dementieva, despite dominating the opening set.
The American won the first seven points, held off two break points at 2-0 and another pair at 3-1 before breaking a second time to lead 5-1.
Her aggressive play often forced Dementieva into errors, but Williams was stretched in the second set as she broke serve four times only for her opponent to break straight back.
With eight consecutive breaks of serve from 2-2, Williams served for the match at 5-4 and 6-5, and netted a forehand on a match point before being taken to a tiebreak.
"She played a lot of good shots at the right times," said Williams. "I was moving forward a lot more in the first set. In the second I caught a lot more behind the baseline, and I think she also improved her game too."
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