A fired-up Novak Djokovic fought back to beat Austria's Dominic Thiem in his opening match at the ATP World Tour Finals in London.
The Serb, who is trying to regain the world number one ranking from Britain's Andy Murray, won 6-7 (10-12) 6-0 6-2.
Djokovic was warned for unsportsmanlike conduct after losing the first set but won 12 of the last 14 games.
If he wins at least two group matches and goes on to lift the trophy he will return to the top of the rankings.
Murray replaced him as number one last week, and the Scot plays Marin Cilic in his opening match in the John McEnroe Group on Monday.