Former world number one Caroline Wozniacki issued a stark warning she was back to her best Wednesday as she ousted China's former French Open champion Li Na to reach the Pan Pacific Open quarterfinals 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 number one 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 quarterfinals-but still needed a second-set tiebreak to shake off Lucie Safarova 6-2, 7-6 (7/5).