Singapore: Agnieszka Radwanska sent top seed Simona Halep crashing out of the WTA Finals on Thursday with a 7-6 (7/5), 6-1 win which also put a resurgent Maria Sharapova into the semi-finals. But despite her 500th career victory, Poland's Radwanska now faces an anxious wait as she needs the Russian to beat Italy's Flavia Pennetta in straight sets later if she is to enter the semis.

Maria Sharapova. Pic/AFP
Maria Sharapova. Pic/AFP

"All I can do is watch and cheer for Maria," Radwanska said. There are no such complex round-robin calculations left for Halep, whose campaign was ended by Radwanska's defensive skills and experience accumulated over seven appearances at the season finale. The Romanian world number two, last year's runner-up to Serena Williams on debut, made a strong start when she immediately broke Radwanska in the first game.

But the Pole broke back to level the match 3-3. Long rallies led to a tie-break that Halep looked set to wrap up at 5-1 until Radwanska, seeded fifth, made spectacular saves at the net and went on to clinch it 7-5. "That definitely was a crazy set," she said. "I was pretty relaxed and still fighting, still playing. I think when you (are) really relaxed, I think it's sometimes better, as we see," Halep added.

The second set became a lopsided affair as Radwanska played with renewed confidence and the now demoralised Halep's energy flagged. "I was done. No energy anymore. I was tired. I felt that I lost the chance to win the first set and probably I lost the chance to win the match in that moment," Halep said. She insisted she was still happy with her injury-marred 2015 season and looked forward to a holiday. Halep retired at the China Open with a left Achilles tendon problem going into Singapore.

Williams, who won a hat-trick at the WTA Finals last year, opted to skip Singapore to rest after losing at the US Open, leaving the field wide open. The WTA Finals features the world's top eight players split into Red and White round-robin pools. The top two in each pool will reach the semis, with the group winner facing the runner-up in the other to determine the finalists.

Spain's Garbine Muguruza is on top of White Group, which also includes Germany's Angelique Kerber and Czech players Petra Kvitova and Lucie Safarova. Final standings in the group are to be determined on Friday.