Shenzhen, China, 1 November 2019 | Darren Walton (AAP) / tennis.com.au

World No.1 Ash Barty scored a crushing win over Petra Kvitova to book her place in the semifinals of the WTA Finals in Shenzhen.

Barty eliminated Kvitova with a decisive 6-4 6-2 win over the dual Wimbledon champion.

Earlier on Thursday, Sam Stosur and Zhang Shuai sealed their place in the last four of the doubles competition in Shenzhen.

In a must-win match for the Aussie-Chinese combo, Stosur and Zheng beat Czechs Barbora Krejcikova and Katerina Siniakova 6-3 7-6(7) to finish the group stage with two wins from three matches.

Stosur and Zhang, the Australian Open champions, trailed 5-1 in the second set and saved a staggering seven set points before going on to triumph.

Rebounding from her previous round-robin loss to Kiki Bertens, Barty delivered the most impressive serving display of the WTA Finals thus far to top the Red Group in Shenzhen with a 2-1 record for the week.

The first eight matches of the tournament had produced an extraordinary 65 service breaks — at an average of one every three games — on the notoriously slow purple indoor court.

But, clutch under pressure, Barty was unbroken on Thursday night, winning 79 per cent of points after landing her first delivery and firing down six aces.

And she needed to be, with Kvitova playing typically aggressive tennis in her desperate attempt to remain in the semifinals hunt.

Attacking at every opportunity, the Czech pounced on anything remotely short, especially second serves, winning 61 per cent of rallies after Barty faulted.

Barty stayed cool, breaking Kvitova in the fifth game of the opening set and twice more in the second, which was enough to give Australia’s top seed her third straight win over the world No.6.

“I felt like I executed really well tonight,” Barty said.

“I knew I had to play aggressively and it was important when I was serving to keep my nose in front.

“It’s really exciting to have another chance to play again here on this beautiful court.”

Barty had lost to Kvitova in the pair’s first four encounters, including in straight sets in this year’s Australian Open quarterfinals.

But Barty is different player nine months on, a Grand Slam champion with the year-end world No.1 ranking all wrapped up after a stellar 2019 campaign that has already yielded three titles from five finals and almost US$7 million in prize money.

The unprecedented windfall for an Australian player in a single season has more than doubled her entire career earnings.

And there could be much more to come after Barty became the first Australian since Stosur in 2011 to qualify for the semifinals at the season finale.

Sam Stosur and Zhang Shuai come back from 1-5 down in the set and save set points in both the set and tiebreak to defeat Krejickova/Siniakova 63 76 and advance out of the Purple Group @WTAFinals. pic.twitter.com/MmKtFRR3XT

— WTA Insider (@WTA_insider) October 31, 2019

The 23-year-old won’t learn who she plays on Saturday until after Friday night’s final two round-robin matches from the Purple Group.

Her possible opponents are defending champion Elina Svitolina, Wimbledon winner Simona Halep or world No.2 Karolina Pliskova.

Bencic plays Bertens in Thursday night’s second match to determine the other semifinalist from Barty’s group.