Venus Williams produced a display of remarkable resilience as she got off the mark at the WTA Finals and left Jelena Ostapenko on the brink of elimination with a hard-fought 7-5 6-7 (3-7) 7-5 victory in Singapore on Tuesday.
World number five Williams was blitzed by Karolina Pliskova in her White Group opener and had to battle for a win over the French Open champion in a match that saw just 16 holds of serve and lasted three hours and 13 minutes, making it the second-longest in Finals history.
Ostapenko was punished for struggling to consolidate breaks in her first outing against Garbine Muguruza and the story was the same as her lacklustre first serve was pounced upon by the seven-time major winner.
The Latvian played to the limit but was guilty of committing too many unforced errors as Williams' experience shone through, and victory for Pliskova later on Tuesday will result in the 20-year-old's elimination.
Ostapenko seized the initiative in the opening game before a double fault immediately restored parity, something she repeated after fighting from 40-0 down to break in game five.
A phenomenal backhand return across the court gave the world number seven a chance to serve out for a 1-0 lead, but Williams showed her experience and broke to love before a long backhand from her opponent gave her the first set.
A backhand down the line into the net was followed by a wild forehand from the Latvian to put the 37-year-old a break up in the second. However, a double-handed backhand found the corner to cancel it out.
The seemingly rejuvenated French Open champion looked set to move clear herself, but once more an errant shot proved costly as Williams got back on level terms.
A thunderous forehand gave Ostapenko a chance to serve for a decider that she again passed up, though she was first to hold in the tie-break and took the match the distance.
The topsy-turvy nature of the match continued - the duo had one hold apiece after 18 break points in the first eight games of the third - until the clearly emotional 20-year-old battled from 40-0 down to hold on a three-deuce game.
It failed to provide the momentum she wanted, though, as Williams collected the 20th break of an astonishing match, before serving out for the victory - converting her second match point.
Williams  bt Ostapenko  7-5 6-7 (3-7) 7-5
Williams - 26/29
Ostapenko - 48/40
Williams - 4/10
Ostapenko - 2/13
BREAK POINTS WON
Williams - 11/26
Ostapenko - 9/18
FIRST SERVE PERCENTAGE
Williams - 62
Ostapenko - 46
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Williams - 61/33
Ostapenko - 51/40
Williams - 141
Ostapenko - 126