SINGAPORE - Garbine Muguruza continued her dream debut at the WTA Finals by completing round robin play with a perfect 3-0 record to advance to the semi-finals after a hard fought 6-4, 4-6, 7-5 victory over Petra Kvitova on Friday.

By topping the White Group, the Spaniard set up a showdown against Poland's Agnieszka Radwanska while Kvitova (1-2) received a huge boost from her Czech Fed Cup team mate Lucie Safarova (1-2), who beat Angelique Kerber to help her advance.

Germany's Kerber (1-2) only needed to win a set in Friday's second match to move on but the already-eliminated Safarova (1-2) found her best form of the week to blast the German off the court and out of the eight-woman event with a 6-4, 6-3 win.

Kerber's loss was Kvitova's gain and the double Wimbledon champion claimed second place in the group courtesy of sets won to book a last four berth against Red Group winner Maria Sharapova.

While her rivals were left scratching their heads and fretting over the various permutations, Muguruza was spared the agony of relying on others by claiming a third straight triumph. "It's amazing to reach the semi-finals," Wimbledon finalist Muguruza said in a courtside interview. "To be here is a great payoff for an amazing year and hopefully I can continue winning and playing like this."

The Spanish right-hander had grown accustomed to facing left-handers in a group containing a trio of southpaws and the recent China Open winner edged ahead in the middle of a tense first set then raced through the eighth game to move 5-3 up.

Kvitova sent down back-to-back aces to ensure Muguruza would need to serve for the set and the world number three duly obliged to claim the set she needed to ensure she would at least advance from the pool as a top-two finisher.

The second set turned into a war of attrition as both players coughed up point after point on their fragile second serves to exchange six breaks in the first seven games before Muguruza ended the sequence to hold in the eighth.

The 22-year-old dropped her racket and let out a yelp of pain when she turned her ankle in the ninth game and while her movement appeared unaffected, her accuracy faltered as she allowed Kvitova to level the match with another break.

Both players continued to struggle on serve as the pair racked up a match total of 15 combined breaks between them, the last arriving in the 11th game of the deciding set when Muguruza moved 6-5 ahead on Kvitova's backhand error. Despite finally gaining an advantage, Muguruza needed to dig deep to prevent the contest from going to a third set tiebreak, maintaining her composure to seal the contest on her fourth match point with a backhand volley.

The second match followed a similar pattern to the first as both players struggled in an early exchange that saw Safarova edge ahead by getting the better of three consecutive breaks and she rode the advantage all the way to a one-set lead.

Safarova's serve improved in the second set and she was able to apply pressure on a visibly frustrated Kerber, who was having more success bashing her racket against the ground than she was making clean contact with the ball. The Czech never looked threatened in the second, breaking early and again the ninth game to seal victory on her fourth match point when Kerber sent a forehand into the tramlines as both players bowed out of the tournament. "That's teamwork," Safarova said of the win that sent her compatriot through to the last four. "I am happy for her that she went through to the semis and I am happy for my win today as well."

Distracted Kerber unhappy:

Germany's Kerber criticised the format of the WTA Finals after she became distracted by qualifying permutations and exited the lucrative end-of-season event following a straight sets defeat by Lucie Safarova.

"I think it's not fair, because I think it's like in the football. Like when it's really counting for something, you should play like the same time," the 27-year-old German told reporters. "I was actually trying to be in my tunnel and just focusing on my match. I told everybody I'm not counting. I will go out and play my match. They were counting. 'Okay, she won one set, now you must win just one set'."

Final round matches in soccer tournaments are played at the same time to ensure all teams chase the win, while avoiding the possibility of opponents making pacts to guarantee qualification. Soccer's world governing body FIFA adopted the policy after West Germany beat Austria 1-0 at the 1982 World Cup which meant both sides went through at the expense of Algeria.

"I think it's fairer like in football if you play in the same time," Kerber said. However, the Singapore Indoor Stadium, in its second year of a five-year deal to host the elite eight-woman WTA Finals, only has space for one court.

Kerber, who won four titles this year, said she understood the limitations but felt the idea should be considered. The German, who beat Kvitova earlier in the week but lost to Muguruza, was more frustrated with her failure to deliver when it was needed most.

"I mean, you have days like that. I will just try to forget the match as soon as possible," she said. "At the end, I had amazing week here. I will just take the positive things to the next week, to my holidays, and, yeah, try to improve my game and everything for the next year."