Rafael Nadal fought back from 5-1 down in the final set to beat Daniil Medvedev 6-7(3) 6-3 7-6(4) to stay alive at the ATP finals in London.
The 33-year-old looked beaten as 23-year-old debutant Medvedev broke serve twice in the decider to stand on the brink of a first career victory over the Spaniard he pushed to five sets in an epic US Open final in September.
For once world number one Nadal's trademark fighting spirit seemed to have ebbed away but he saved a match point at 1-5 with a superb drop shot and was suddenly re-energised.
Twice Medvedev served for the match, at 5-2 and 5-4, but he tightened up and world number one Nadal, roared on by the 02 Arena crowd, needed no second invitation to take advantage.
"I had to win this match and it's good for my confidence," said Nadal court-side after the match.
" Lucky for me, not so for Daniil. Some good moments from myself at the end, but in general I know I want to be able to be in a better way towards the end of the tournament. "
"I'm very happy for this victory."
Wednesday afternoon's showdown was must-win for both men, following losses for Nadal and Medvedev against Alexander Zverev and Stefanos Tsitsipas respectively in their group openers.
Nadal, whose preparation for the season-ending showpiece has been blighted by an abdominal injury, fought back from a set down to force a decider but an out-of-sorts Nadal was broken twice as the Russian opened a four-game advantage.
However, Nadal, ranked number one in the world, rallied, securing two break-backs and, in doing so, fended off match point at 5-1 to send the match to a final-set tie-breaker.
And the 33-year-old, having struggled for large swathes of the contest and the tournament, found an extra gear to seal an extraordinary comeback to take the first of two match points on offer.
Nadal has never won the ATP Finals title, the only gap on a CV that includes 19 Grand Slam titles and Olympic gold.