Tokyo - Former world No 1 Caroline Wozniacki issued a stark warning she was back to her best on Wednesday as she ousted China's former French Open champion Li Na to reach the Pan Pacific Open quarter-finals in Tokyo.
Wozniacki ground out a 4-6, 6-3, 6-4 win to join a powerful last eight, where she will play defending champion Agnieszka Radwanska. Maria Sharapova, Samantha Stosur and world No 1 Victoria Azarenka are also through.
Denmark's Wozniacki won her first WTA title in more than a year last week in Seoul, and she was not to be denied despite going a set down against Li. Showing trademark grit, she toughed it out in the last set for her eighth straight win.
"After the first set of my first match my energy level dropped, but since then I've been feeling great," said the 2010 winner, who is currently ranked 11th.
"Today, Li Na was playing very close to the baseline and pushing me away, taking the balls very early - she was all over me. But in the second set I really started to step in and push her away."
Li's presence had attracted attention after other Chinese athletes opted to stay away from Japan over a flare-up in a territorial dispute between the rival Asian powers.
Elsewhere Sharapova found improved form to reach the quarter-finals - but still needed a second-set tiebreak to shake off Lucie Safarova 6-2, 7-6 (7/5).
The French Open champion from Russia, who needed more than three hours to beat British qualifier Heather Watson on Tuesday, admitted frustration at letting the Czech player back into the match.
"After winning the first set, I was up 3-0 in the second set. I didn't think it was going into a tie-break," said Sharapova, 25, who won the Tokyo tournament in 2005 and 2009.
"She had an opportunity to serve out for the second set and I was frustrated in that situation."
Six games in the first set went against serve, and both players struggled to hold at the end of the second set as it went to a tiebreak. Sharapova took a 3-0 lead before being pegged back to 4-4, but finally clinched it 7-4 with an ace.
"After the long match yesterday, I'm very happy to be able to win today. I was a little bit down in the second set and I'm just happy to win in two sets," Sharapova said.
In the quarter-finals on Thursday, Sharapova will play eighth seed and former US Open champion Samantha Stosur of Australia, who defeated Dominika Cibulkova of Slovakia 6-4, 7-5.
Belarusian top seed Azarenka outlasted Roberta Vinci of Italy 6-4, 6-2 to set up a meeting with Angelique Kerber, while last year's winner Radwanska, the third seed, defeated qualifier Jamie Hampton of the United States 6-4, 6-3.
Sixth seed Sara Errani of Italy outclassed ninth seed Marion Bartoli of France 3-6, 6-2, 6-2 to set up a quarter-final with Nadia Petrova of Russia, who beat 73rd-ranked Croatian Petra Martic 7-6 (7/2), 6-7 (1/7), 6-4.