Agnieszka Radwanska felt she raised her game when it mattered most to beat Simona Halep in straight sets and reach the last four of the WTA Finals.

Radwanska had only a slim chance of securing a place in the semi-finals ahead of her final Red Group match against Halep, needing a straight-sets win and Maria Sharapova to do her a favour by beating Flavia Pennetta by the same margin later in the day.

The Pole dumped out top seed Halep with a 7-6 (7-5) 6-1 victory, then watched on as Sharapova saw off US Open champion Pennetta 7-5 6-1.

Radwanska's triumph over Halep was the 500th of her career and the fifth seed was able to reflect on a memorable day in Singapore.

She said: "I think I was really playing great tennis today. That was I think much better tennis than the other days – It was really good quality tennis the whole match.

"I knew I had to win to really stay alive and I think I played some great tennis, especially in the important moments. Of course there were some crazy ups and downs but I am very, very happy."

Radwanska also gave credit to Halep, who bows out a year after being runner-up to Serena Williams.

"The first set was key. Against Simona is never easy, she's a great fighter and there are no free points – I was feeling much better in the second set, everything was working for me and I'm just very happy I could win that match."