Red-hot world number three Karolina Pliskova has vowed to keep her foot on the accelerator after crushing Garbine Muguruza on Tuesday to become the first semi-final qualifier in the WTA Finals.

Pliskova, who is in contention to finish world number one, continued her stranglehold over the Spaniard with a 6-2, 6-2 victory in just 62 minutes. The Ukrainian has not dropped a set from her two games to ensure she finishes top of the White Group.

She will play her final round robin match on Thursday against winless number seven seed Jelena Ostapenko, who has been eliminated.

Even though the match against Ostapenko will be a dead rubber, Pliskova said she wanted to continue her rich vein of form.

"Maybe it's going to have a little bit of a different atmosphere but... I don't want to lose any of these matches I'm playing," she told reporters after the match.

"It would be nice to win all three (matches) in the group. If I can win a few matches in a row, it will give me extra confidence."

It was her seventh victory from nine starts over the world number two, who appeared hampered by an upper leg injury, which required strapping early in the match.

The result means Muguruza and Venus Williams, in a re-match of the Wimbledon final this year, will fight for the last semi-finals spot from the White Group when they clash on Thursday.

Muguruza said the strapping to her left thigh was "just prevention" and she lamented her performance.

"I think my tennis wasn't there tonight. I didn't start well," she said, adding she was "a little bit disappointed".

Pliskova came out firing with her trademark serve proving lethal. She had a 70 percent success rate with her first serve in the early going and relentlessly targeted Muguruza's body.

The 25-year-old cruised through the first set in just 26 minutes and the one-sided rout continued. Pliskova won the first five games of the second set before Muguruza saved some face by winning consecutive games.

But it wasn't long until Pliskova deservedly sealed her spot in the final four.

Earlier, Williams revived her WTA Finals campaign with an epic three-hour, three-set victory over Ostapenko.

The 37-year-old world number five rebounded from her opening defeat to Pliskova with a marathon 7-5, 6-7 (7/3), 7-5 win over three hours and 13 minutes –- just one minute shy of the record played at the Singapore Indoor Stadium.

The 37 games played was a WTA Finals tournament record.

"A win is a win. That's all I can say," a terse Williams told reporters after the match.

The result will sting for Ostapenko who started brightly, working Williams around the court and hitting the lines with precision, notably several piercing cross-court winners.

The 20-year-old delivered 12 of the first 13 winners in the match before her confidence nosedived as she struggled with her serve, which was broken nine times by Williams.

Ostapenko admitted problems on her serve brought about her undoing.

"I was just a little bit tight in some moments," she said. "I fought until the last point but my serve wasn't really amazing today."

Source: AFP