SINGAPORE: Agnieszka Radwanska rallied past an exhausted Garbine Muguruza to book her place in the WTA Finals title decider on Saturday with a 6-7(5) 6-3 7-5 victory over the Spanish prospect in a classic encounter.

The Pole used all of her guile and defensive skills to see off the powerful 22-year-old, setting up a showdown against either Maria Sharapova or Petra Kvitova in her first final at the season-ending event in her seventh appearance.

"I didn't know I could come back after that first set. It was a great match from the beginning to the end, lots of ups and downs, so many rallies and a lot of running. I am just so glad I could win that match," Radwanska said in a courtside interview.

The first set was a slow burner that turned into a thriller once both women had settled into a rhythm but the longer the contest progressed, the stronger Radwanska became against an opponent who is also playing the doubles event in Singapore.

Muguruza was facing her first right-handed opponent of the week and she took to the task with aplomb, breaking in the first game and racing to a 40-15 lead in the second, before a confidence-sapping sloppy run of points stalled her progress.

Radwanska worked her way into the contest after capitalising on the Spaniard's wayward play, reeling off four straight games and appearing likely to ride the momentum to a one-set lead before Muguruza came storming back to tie the scores at 4-4.

By now, both women were playing excellent tennis and the high-class encounter moved inevitably towards a tiebreak as the service winners increased and error count dropped.

FIST PUMP

Radwanska raced to a 3-0 lead in the tiebreak but Muguruza rallied again to surge ahead with some aggressive play and sealed the opener on her second set point when the Pole found the net at the end of a brilliant rally.

Muguruza greeted the decisive point with a fist pump and a roar but her play at the start of the second set belied her positive body language as Radwanska forced a plethora of errors to race to a 4-0 lead that she never looked like relinquishing.

A mini-revival in the middle of the set brought Muguruza to within one game but Radwanska pulled away again and levelled the contest with her third break of the set.

The exertions of her first appearance in the WTA Finals were clearly visible on Muguruza, who appeared unable to even hit the ball over the net in her opening service game of the decider and was broken easily by a rampant Radwanska.

Yet somehow and from somewhere Muguruza found the energy to rally from 4-1 down in the decider to put the contest back on a knife-edge before Radwanska was finally able to put her opponent away with a decisive break in the 12th game of the third set.

"I have nothing to lose tomorrow so it doesn't really matter to me who wins tonight, I will just try and enjoy my game tomorrow and hopefully I can play as well as I did today," the Pole said after completing victory in two hours, 41 minutes.