SINGAPORE - World number three Karolina Pliskova booked a berth in the semi-finals after delivering a masterclass to overwhelm Garbine Muguruza in a straight sets demolition in the WTA Finals on Tuesday.

Pliskova continued her stranglehold over the world number two with a 6-2, 6-2 victory in just 62 minutes. It was her seventh victory from nine starts over the Spaniard, who appeared hampered by an upper leg injury, which required strapping early in the match.

Pliskova's victory means she will finish top of the White Group and knocks out winless number seven seed Jelena Ostapenko. Muguruza and Venus Williams will fight for the last semi-finals spot from the White Group when they clash on Thursday.

Pliskova came out firing with her trademark serve providing 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.

The round robin match was strewn with no fewer than 20 breaks of serve, with Williams capturing 11 of them -- enough to edge her to a victory that ensures she stays in the season finale. It was relief for the American, who let slip a match point in the second set but summoned up all her experience to stay cool when she got another opportunity in the third.

During the 69-minute third set, which included six consecutive breaks, Williams clinched the decisive break in the 11th game and then clinically held serve to outlast the Latvian.

It maintained Williams' perfect record over Ostapenko, having won their only encounter earlier this year at Wimbledon. 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. Even though she had a major 48-26 advantage in winners, Ostapenko had only a 46 percent success rate with her first serve, and dished up 13 double faults.