In the second White Group encounter of the day, Elina Svitolina moved to the brink of the semi-finals with a gutsy 6-3, 2-6, 6-3 win over Karolina Pliskova.
Caroline Wozniacki overcame a mid-match wobble and withstood a barrage of big hitting to get her WTA Finals title defence back on track with a 7-5, 3-6, 6-2 victory over Petra Kvitova at the Singapore Indoor Stadium on Tuesday.
In the second White Group encounter of the day, Elina Svitolina moved to the brink of the semi-finals with a gutsy 6-3, 2-6, 6-3 win over Karolina Pliskova, the Ukrainian recording her second straight win of the tournament to top the pool.
Wozniacki and Kvitova came into their contest on the back of opening defeats in round robin play but the Danish world number three ended a run of four straight losses to the Czech to stay alive in the eight-woman tournament for at least one more match.
The Dane, who started her campaign with a 6-2, 6-4 loss to Pliskova on Sunday, spent most of the match against Kvitova pinned on the baseline but her consistency and patience proved vital as she pulled away in the final set.
"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."
The match was a cagey affair from the outset with seven holds of serve in a row. But after they traded breaks as the intensity levels picked up, Wozniacki broke again to seal the opener when Kvitova blasted a forehand into the net.
Kvitova took control of the second set with an early break, then lost momentum when Wozniacki took a timeout for treatment on her knee, before regaining the upper hand to level the match with a sliced backhand winner on her third set point.
Sensing her title defence was hanging by a thread, Wozniacki regrouped to break at the start of the decider when she put away a backhand to win the longest rally of the match, and ran away with the contest as Kvitova wilted.
The 28-year-old Wozniacki sealed victory on her first match point with a big serve that Kvitova sent long, and will hope to complete round robin play with a victory over Svitolina to secure a semi-final berth.
Svitolina was a bundle of energy at the start of the second match, full of intensity and covering every inch of the court as she rode her early momentum to wrap up the opening set with a forehand winner that was greeted with a big clenched fist.
The Ukrainian had played almost perfect tennis until midway through the second set when a combination of good fortune and rasping cross-court winners earned Pliskova her first break of the tie, a feat she repeated two games later to level the match.
Svitolina shrugged off the setback to come storming out of the blocks in the decider, racing to a 4-0 lead that she carried all the way to victory despite a late fightback from Pliskova.
She only needs to take a set off Wozniacki on Thursday to reach the last four for the first time but as it stands, all four players can still mathematically reach the semi-finals.
Federer survives scare
Top seed Roger Federer showed signs of ring rust in his first match at the Swiss Indoors on Tuesday but still had enough in the tank to push past unseeded Serbian Filip Krajinovic 6-2, 4-6, 6-4 and reach the last-16 in Basel.
Playing in front of his home fans Federer struggled to find his usual precision, racking up 38 unforced errors, but still managed some produced his usual array of dazzling winners a contest which stretched just past the two-hour mark.
Next up for the world number three, an eight-time Basel champion, is Germany's Jan-Lennard Struff, who beat Australian John Millman 7-6(3), 6-2.
In-form Russian seventh seed Daniil Medvedev also reached the last 16 by defeating German Maximilian Marterer 6-3, 7-5.
Medvedev, winner of this month's Japan Open as a qualifier, faces unseeded Italian Andreas Seppi next after he served up the day's only bagel en route to beating Japanese qualifier Taro Daniel 6-0, 6-4.
Australian teenager Alexei Popyrin claimed his first main draw victory on the ATP Tour as the qualifier got the better of a tussle with compatriot Matthew Ebden, winning 7-6(4), 6-4.
His prize could be a clash with second seed Alexander Zverev if the German gets past Dutchman Robin Haase.