An improved strength and conditioning programme has driven Ana Ivanovic's tennis game to new heights, the Serbian world number three said on Monday.

Ivanovic, who has been working with a fitness coach for 18 months, reached the Australian Open quarter-finals after using her new-found stamina to beat a stubborn Caroline Wozniacki 6-1, 7-6 in the fourth round.

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"My fitness ... definitely needed improvement," Ivanovic told reporters. "I wasn't a great mover and I had problems with injuries.

"So when I started working with him I got really fit. I lost some weight, and got really stronger, especially in my upper body.

"Now I feel much stronger out there and I feel I can keep up with these girls for a longer period of time.

"Before I had to go for my shots earlier in the rally because I couldn't keep in the rally."

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Ivanovic, who has not dropped a set in her march to a quarter-final showdown with eighth seed Venus Williams, dominated the first set by smashing 15 winners to Wozniacki's none and winning 76 percent of her first service points.

After the pair held for two service games each in the second set, the 17-year-old from Copenhagen finally broke in the fifth game and held two set points in the 10th, but Ivanovic broke to level at 5-5.

The Serb held serve and had a match point on Wozniacki's, but the young Dane managed to force a tiebreak.

However, Ivanovic's experience told and she ran out a 7-2 tiebreak winner when Wozniacki's service return failed to clear the net.

"She had nothing to lose, so she was really dangerous in the second set," Ivanovic said.

"I started fighting a little bit more ... [and] I'm really happy I managed to keep my composure in the end of that second set and be calm on those set points and still manage to win it."

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