Karolina Pliskova dug deep to overcome a stubborn Garbine Muguruza and start her WTA Finals campaign with a well-earned victory.
The French Open champion had turned the tide of a dominant early Pliskova performance, but the Czech found a second wind late in the piece to round out a 6-2 6-7 (4-7) 7-5 win.
Fourth seed Pliskova was quick out of the blocks, conceding just seven points on her booming serve, while making routine in-roads onto Muguruza's delivery to take the first set in a little over half an hour.
The Czech immediately broke in the second set as a simple victory looked on the cards, but Muguruza stuck to her guns, holding position behind the baseline, with an increased use of slice throwing Pliskova off her game somewhat.
A scruffy second set went to a tie-break when Pliskova again seized an instant advantage, but Muguruza roared back from 3-0 down, winning seven of the next eight points to blast her way to parity.
And the momentum shift continued into the decider as Pliskova began to look tired and her serving power dwindled, Muguruza racing into a 4-0 lead.
But a 15-minute long sixth game, resulting in a Pliskova break, stunted the Spaniard's progress and inspired a stunning comeback and the Czech smashed a string of tired Muguruza serves beyond her to take match point at the first opportunity at 01:22 local time (18:22 BST).
Victory puts Pliskova level with Svetlana Kuznetsova in the White Group after the Russian's epic near three-hour win over defending champion Agnieszka Radwanska.
Pliskova  bt Muguruza  6-2 6-7 (4-7) 7-5
Pliskova - 34/37
Muguruza - 36/43
Pliskova - 14
Muguruza - 2
BREAK POINTS WON
Pliskova - 8/15
Muguruza - 5/9
FIRST SERVE PERCENTAGE
Pliskova - 61
Muguruza - 56
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Pliskova - 66/40
Muguruza - 60/39
Pliskova - 116
Muguruza - 106