Wozniacki claims first win over a world No 1 to reach Toronto semi-finals

World No 6 from Denmark will take on American Sloan Stephens for a place in the final.

Caroline Wozniacki beat a world No 1-ranked player for the first time in her career following six attempts. Warren Toda / EPA

Caroline Wozniacki survived three rain delays to book her spot in the Canadian Open semi-final with a marathon 7-5, 6-7 (3/7), 6-4 victory over world No 1 Karolina Pliskova on Friday.

The sixth seeded Wozniacki posted her first career win over a No 1 player in seven tries as she blasted eight aces in the two-hour, 56-minute centre court match.

She next faces American Sloane Stephens, who saved three match points in the third set to oust Lucie Safarova 6-2, 1-6, 7-5 in their quarter-final. Pliskova fired five aces but won just 45 per cent of her second-serve points.

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"I thought she started off really strongly and she was attacking and playing really, really well," Wozniacki said. "After that, I feel like I stepped a little bit closer to the line just to try and take away a little bit of time. I once I kind of got on a roll, I started serving better.

"I just tried to stay steady, tried to take the ball on the rise and try to stay aggressive when I could and keep my serves aggressive too."

Wozniacki's record against No 1s included five losses to Serena Williams and one to Dinara Safina.

"I don't really think about too much," Wozniacki said, speaking of her feat in defeating a top seed. "I knew I was going to play someone that was really tough today, and I'm just happy with my win."

Pliskova started quickly by winning five of the first six games of the first set which was delayed at the start by rain. Wozniacki then started to assert herself on defence and offence, moving Pliskova around the court to win six straight games to take the first set.

Pliskova never recovered from that first set loss. From then on, it was a matter of Wozniacki wearing her opponent down by keeping the ball in play.

"I think the biggest mistake was in the first set - so because of that, I think I lost," Pliskova said. "Overall, the match was much longer than just the hours on the court.

"It's just tough to play her in these conditions. She's putting everything back, so obviously, I did some mistakes."

Wozniacki will have her hands full with Stephens, who reached the quarters with wins over third seed Angelique Kerber and two-time Wimbledon champion Petra Kvitova.

Stephens won the last four games and saved three match points to oust Safarova.

The other semi-final line-up is yet to be decided due to the rain limiting play on Friday.

Wimbledon champion Garbine Muguruza starts Saturday leading Ukraine's Elina Svitolina by one set having taken the opener 6-4. World No 2 Simona Halep is yet to start her quarter-final against Frenchwoman Caroline Garcia.

* Agence France-Presse