Germany's Alexander Zverev punished Croatian rival Marin Cilic's inability to take his chances and opened his ATP Finals campaign with a 7-6 (7-5) 7-6 (7-1) win at the O2 Arena in London.
Cilic, 30, served for the first set but made three unforced errors and a double fault as Zverev broke back for 5-4.
Zverev, 21, went on to take the tie-break and then fought back from a break down midway through the second set.
Cilic saved a match point at 5-4, but Zverev clinched victory in the breaker.
Zverev, like he did in the first-set tie-break, raced into a comfortable lead in the second decider, taking the second of five more match points with a serve out wide which Cilic could not return.