By John O'Brien

SINGAPORE (Reuters) - Poland's Agnieszka Radwanska stayed patient in the face of a barrage of big serving to advance to the last four of the WTA Finals for the third time in a row with a 7-5 6-3 victory over Karolina Pliskova on Friday.

The defending champion came into the contest with a 6-0 record against the powerful Czech and maintained her dominance over last month's losing U.S. Open finalist to set up a mouth-watering semi-final against world number one Angelique Kerber.

Earlier in the day, Garbine Muguruza ended her campaign with a 3-6 6-0 6-1 consolation victory over Svetlana Kuznetsova, who still emerged as the White Group winner and will face Dominika Cibulkova in the other semi-final on Saturday.

After compiling identical 1-1 records in their first two round robin matches, both losing to Kuznetsova but beating Muguruza, Friday's contest was a winner-takes-all affair and Radwanska kept her composure on the key points to advance.

"It was a very tight match and I was happy I could do my best at the important moments," said world number three Radwanska, who has won 14 sets without reply against Pliskova.

"You just wait and pray that her first serve isn't going to be in and take you chances on the second serve. She's one of the best servers on the tour so I knew it was going to be tough."

Pliskova started strongly, racing through her service games with ease and putting pressure on Radwanska, who was broken in the fifth game to fall 3-2 behind.

COMPELLING TENNIS

Radwanska was creaking but after staving off another threat, the defending champion capitalised on Pliskova's first loose service game of the contest to tie the set at 4-4.

As her first serve percentage improved, the Pole held her next two games with relative comfort and as Pliskova's form dipped a fraction, Radwanska pounced to break again and seal the set with a sumptuous return winner.

While the tennis was not of the highest quality it was certainly compelling, and with both players struggling to hold, the second set remained in the balance until a pair of failed smashes handed Radwanska a break in the eighth game.

The Pole immediately sent down her second ace of the match to set her on the way to victory, which she sealed with a deep forehand that her fifth-ranked opponent could only blast into the net.

Kuznetsova started her previous two matches slowly but she was quick out of the blocks against Muguruza, breaking the Spaniard in the third game to move ahead before she repeated the feat in the ninth to wrap up the opener in 29 minutes.

The Russian had been fairly untroubled on her serve, not even taken to deuce in the opener, but her radar was off-target at the start of the second set when a second double fault gifted Muguruza a break in the first game.

The French Open champion was suddenly back in the contest as Kuznetsova appeared to lose her range and focus, the tall Spaniard breaking twice more while holding comfortably to race through the set without dropping a game.

Muguruza carried the momentum into the decider, breaking immediately and staving off seven break points as Kuznetsova ran out of steam, the 23-year-old Muguruza winning 11 of the last 12 games to seal victory.

(Editing by Nick Mulvenney, Simon Cambers)