Svetlana Kuznetsova dug deep into her reserves to overcome Agnieszka Radwanska 7-5, 1-6, 7-5 in their WTA Finals round robin opener on Monday, the late-arriving Russian upsetting the defending champion in a thrilling encounter.
In the second White Group tie of the day, Czech Karolina Pliskova outlasted Spain's French Open champion Garbine Muguruza 6-2, 6-7(4), 7-5 in another marathon match that finished at 1:21 a.m.
Kuznetsova only qualified for the elite eight-woman tournament on Saturday after winning the Kremlin Cup but she staved off exhaustion and a match point in the deciding set to emerge victorious over the world number three.
"At some point I was ready to just let it go and lie on the court and let them take me out of here but I decided to hang in there," the two-time grand slam champion said.
The World No 9 held a 12-4 record against Radwanska prior to the contest but after travelling from China to Russia to Singapore in consecutive weeks, few gave her a chance of recording an upset victory on Monday.
Both players looked comfortable on serve in the opening exchanges before Radwanska pounced in the fourth game, finding the corners with her backhand against a leaden-footed Kuznetsova to forge 3-1 ahead with the first break of the match.
The Russian was struggling to gauge the pace of the slow hardcourt at the Singapore Indoor Stadium, her shots finding the net with alarming regularity, but she survived another threat in her next service game to stay just one break down.
The 31-year-old was growing into the contest, moving better and offering more resistance, and her improved play was rewarded in the seventh game when she capitalised on some sloppy play by the Pole to break serve before tying the set up at 4-4.
Kuznetsova continued to press and probe the world number three's notoriously resilient defence, breaking once more thanks to a backhand winner and crosscourt pass before sealing the set when Radwanska plonked a return into the net after 50 minutes.
Radwanska adopted a more aggressive approach to start the second set and the strategy paid off as she began to dictate a majority of the points, racing to a 5-1 lead on three breaks of serve and sending the match into a decider with a gutsy hold.
The Pole carried her momentum into the third set, continuing to move the Russian around with her sliced backhand to edge ahead with an early break but Kuznetsova, who gave herself an impromptu haircut at the changeover, broke back to make it 2-2.
By this stage, there was nothing between the pair with Radwanska and her weary-looking opponent unable to hold serve as the tension mounted but it was Kuznetsova who got the decisive break to seal victory after two hours and 48 minutes on her third match point.
In the late encounter, Pliskova became the second player of the day to stave off a match point and win when the beaten US Open finalist rallied from 5-2 down in the deciding set to grind down Muguruza in a titanic tussle between big servers.
Pliskova raced through the first set and was a break up in the second before the Spaniard roared back and had two chances to hold serve to claim victory but the Czech rediscovered her form just in time to reel off five straight games.