It has been missing for a while but Novak Djokovic rediscovered his warrior spirit to tame Canada's Milos Raonic 7-6(6) 7-6(5) in a rivetting duel and guarantee progress from his group at the ATP World Tour Finals in London.
The 29-year-old Serb's recent malaise has cost him the world number one ranking he held for 122 weeks but when backed into a corner by an inspired Raonic, he responded in the belligerent manner that has earned him 12 grand slam titles.
World number four Raonic threw the kitchen sink at the elastic-limbed Serb for more than two hours but Djokovic, bidding for a fifth consecutive title at the year-ender and a possible return to top spot, was equal to everything -- just.
Having edged a high-intensity first-set when Raonic double-faulted on set point down in a tense tiebreak, Djokovic looked to have broken the shackles in the second to lead 4-2.
Raonic, serving 140mph bombs, stormed back to rock Djokovic on his heels and came within a missed forehand of extending the match into a late-night decider.
Djokovic looked mightily relieved when Raonic's backhand flew wide on match point and he can now look forward to a semi-final on Saturday, whatever the outcome of his final group match against Gael Monfils on Thursday.
Raonic will face Austria's Dominic Thiem with the victor joining Djokovic in the semis after debutant Thiem claimed his first win at the tournament with a 6-3 1-6 6-4 defeat of Monfils earlier at the O2 Arena.
"I managed to hang in there mentally and stay strong and believed that the opportunities would come and that I could take them," Djokovic, who needs to better Andy Murray's performance here to return to the summit, said on court.
"Milos was hanging in there and only one or two points separated us in the second set. It could have gone either way this match. He's such a strong player and has got firepower."