Ukraine's Elina Svitolina moved one step closer to reaching the final four at the WTA Finals in Singapore by defeating Karolina Pliskova of the Czech Republic 6-3 2-6 6-3.
World No 7 Svitolina followed up Sunday's straight-sets victory over another Czech, Petra Kvitova, to make it back-to-back wins in her White Group matches.
Svitolina out-served her seventh-seeded opponent, firing eight aces to triumph in an hour and 54 minutes.
"It was a tricky match, I tried to stay focus," Svitolina said after the match. "It has been a tough season for me, but the way I'm playing now is really good."
Earlier, defending champion Caroline Wozniacki staved off the threat of an early exit with a three-set victory over Petra Kvitova.
The world No 3 defeated the Czech player 7-5 3-6 6-2 in the second round of matches.
"I played much better today. I served and returned pretty well, and moved really well against a player who is so powerful that you have to stay on your game," Wozniacki said.
"You never know what is going to happen against Petra, so I just stayed focused and got a lot of balls back."
Caroline Wozniacki picked up a much-needed win against Petra Kvitova
After saving a crucial break point in the 11th game of the opening set, the Dane - who lost to Karolina Pliskova in her opening match of the round-robin stage on Monday - made the most of her third set point to take an early lead.
Kvitova was guilty of 16 unforced errors in the opening set but showed resilience by securing four breaks of serve to level at 1-1.
The world No 5 failed to build on the momentum, however, and was convincingly beaten in the decider, leaving her on the brink of elimination following successive defeats.
She could have sealed her semi-final place with another straight-sets win, but Pliskova's second-set fightback ensured the White Group winners will be decided on Thursday.