By Mitch Phillips

With Serena and Venus Williams going out Monday, the women's singles competition looks the most open for years but it is Cibulkova rather than Wozniacki now hoping to take advantage after she showed grit to recover from a first-set drubbing.

She maintained that form in a quickfire first set and few in the crowd would have suspected an upset was on the cards.

However, Cibulkova pulled herself together in the second set and, playing far more aggressively, took the game to the top seed.

Unleashing the huge forehands that belie her diminutive stature, the Slovak 24th seed forced a tiebreak which she took 7-5 to ask the world number one a serious question for the first time in the tournament.

The third set was all over the place with two breaks apiece in the first six games as the two women backed themselves to overpower the other from the baseline.

It then went with serve to 5-5, when Cibulkova again broke.

She held three match points in her final game and though Wozniacki saved two of them, Cibulkova kept her nerve on the third to win the match by fittingly crashing yet another forehand winner down the line.

"Wozniacki is just great, she's the best defensive player, she waits and waits and takes her chance so after the first set I realized I had to go for my shots," said Cibulkova.

"It was all or nothing, that's how I started to play, and I was really going for everything and I played really well today.

"I just had to be more aggressive on return, even in the first set I was doing well in the few rallies there were so I had to get into the rallies.

"I was really risking it, my forehand was really working and I went for it," added Cibulkova, who meets sixth-seed and 2004 winner Maria Sharapova in the quarter-finals.

Wozniacki had no excuses but was hugely frustrated to have missed out on such a good opportunity.

"It's kind of disappointing because when you're up two-love with a break and have a lot of breakpoints that you don't convert, that's kind of your own fault," she said.

"I had a chance and I usually take it. That's why I am where I am. But today it just didn't go my way. She played well, as well so you just have to give credit to her.

"She just goes for everything, she doesn't hold back So, you know, if it goes in, it's tough."

(Editing by Ed Osmond)