Singapore - Agnieszka Radwanska rallied past an exhausted Garbine Muguruza to book a maiden appearance in the WTA Finals title match with a 6-7(5) 6-3 7-5 victory over the Spaniard in a classic encounter on Saturday.
The Pole used all her guile and defensive skills to see off the powerful 22-year-old, setting up a showdown against Petra Kvitova, who stunned Maria Sharapova 6-3 7-6(3) in the second semi-final.
"I didn't know I could come back after that first set," world number six Radwanska said in a courtside interview.
"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," added the Pole, who secured a first final berth at the season-ending event in her seventh appearance.
The first set was a slow burner that turned into a thriller once both women had settled into a rhythm and the longer the contest progressed, the stronger Radwanska became against an opponent who is also playing the doubles event in Singapore.
Radwanska worked her way into the contest after the fast-starting Spaniard wavered, reeling off four straight games and looking likely to ride the momentum to a one-set lead before Muguruza came storming back to force a tiebreak.
The Pole raced to a 3-0 lead but Muguruza recovered to surge ahead with some trademark aggressive groundstrokes and sealed the opener on her second set point when Radwanska 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 mid-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 WTA Finals were now clearly visible on Muguruza, yet somehow she found the energy to rally from 4-1 down in the third set and put the contest back on a knife-edge before Radwanska finally got a decisive break in the 12th game.
The second semi-final pitted two former WTA Finals champions against each other and while it was shorter than the first match, Kvitova's stunning comeback from 5-1 down in the second set was equally as absorbing.
Kvitova and Sharapova, two of the strongest servers on the Tour, traded early breaks before settling into a baseline battle until the Czech edged ahead in the seventh game of the opening set.
The world number five could sniff blood and broke again in the ninth game to take a surprise lead against a player who had won all three of her round-robin ties despite arriving in Singapore short of matches after a long injury layoff.
Kvitova, however, is prone to bouts of erratic play and the confidence she exuded in the opener was replaced by uncertainty as Sharapova, who has added greater variety to her game as she has grown older, raced 4-0 ahead.
The Russian somehow contrived to hand back the double break advantage, even wasting a set point at 5-2, as Kvitova rediscovered her top form to reel off five straight games and force a tiebreak.
The 2011 champion was now in full control, setting up three match points with a booming serve down the middle and when Sharapova sent a forehand sailing over the baseline, Kvitova had completed the most remarkable of second-set turnarounds.