Shenzhen, Oct 30 (AFP) Defending champion Elina Svitolina stormed into the semi-finals of the season-ending WTA Finals after a commanding win over Simona Halep on Wednesday, while Canadian sensation Bianca Andreescu retired hurt against Karolina Pliskova.

Andreescu injured her left knee in the third game after attempting to return a wide serve and received heavy strapping during the subsequent medical timeout.

The 19-year-old gamely fought on but was noticeably hobbled and retired after the first set, which Pliskova won 6-3 in 51 minutes.

"I was really pumped for today but I feel sorry that she had to retire," Pliskova said after the match.

Svitolina will finish top of Purple Group after taking advantage of an error-strewn Halep to win 7-5, 6-3 in one hour and 39 minutes at the Shenzhen Bay Sports Centre.

Svitolina is the first player through to the semi-finals, having also beaten world number two Pliskova in straight sets on Monday. Wimbledon champion Halep will play Pliskova on Friday to decide the group's second spot.

Svitolina had the better of long baseline rallies against an uncharacteristically errant Halep, who finished with 38 unforced errors.

"It's about managing attack and defense for me," Svitolina said.

"I've been working on this. That's working well this week."

After a slow start, Halep came back into the contest and a powerful forehand winner down the line clinched her break in the fifth game before the players held serve. A sloppy Halep misfired in the 12th game as she handed Svitolina the first set with a slew of unforced errors.

Halep started well in the second set with an early break before falling away against a more consistent Svitolina.

The result broke a deadlock between the pair with Svitolina now holding a 5-4 edge and exacting revenge for a semi-loss to Halep at Wimbledon in July.

The 25-year-old is hoping to end a barren season on a high having failed to win a title since last year's WTA Finals triumph.

Halep said she had to sharpen her game to stay in the tournament. "I really want to win because I want to qualify from this group," she said.

"Hopefully I can recover because I'm a little bit tired and play better than today," she added. (AFP)

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