NEW YORK (Reuters) — Boxing is one of top seed Caroline Wozniacki's off-court interests and the 20-year-old Dane has exhibited a fighting spirit during her charge through the draw at the US Open.

Wozniacki had to fight through near gale-force wins and a determined effort from Dominika Cibulkova of Slovakia on Wednesday to advance to Friday's semi-finals.

"Boxing, it's just a different way for me to work out,'' she told reporters after her 6-2 7-5 quarter-final victory.

''It's fun, it's interesting, and it's great because I have this great coach who knows how to get me in good shape and what I need for my tennis, as well.''

Wozniacki, a surprise runner-up at last year's US Open, engages opponents in a war of attrition, matching her groundstroke consistency against theirs and counterpunching to great effect.

Conditioning is a key to her success.

''I wanted to try something different, something that was not the usual things. I just love the training. It's great.

''It's really hard not only physically, but also the conditioning. And, you know, it's great to have a coach there also who just pushes you to the limit,'' she said.

The Dane said the two sports promote some of the same skills.