In beating John Isner 6-4, 6-3, Djokovic lost only six points on his serve the entire match.
LONDON — Novak Djokovic made an impressive start to his campaign for a sixth ATP Finals title by dismissing John Isner 6-4, 6-3 on Monday.
A day after a ceremony to mark ending the season as No. 1, Djokovic underlined his status in a near-faultless display against the American debutant.
Djokovic dropped just six points on his own serve and made only six unforced errors. The Wimbledon and U.S. Open champion nullified Isner's usually destructive serve and could have easily had more than the sole break he managed in the opening set.
Isner, who at 33 became the season-ending tournament's oldest newcomer since 1972, applied himself admirably but was broken twice more in the second set.
Earlier, Alexander Zverev came from a break down in both sets to defeat Marin Cilic 7-6 (5), 7-6 (1).
Facing each other in their opening match for a second straight year, Zverev produced the slightly steadier tennis in an error-strewn contest to extend his record against Cilic to 2-0 at the O2 Arena and 6-1 overall.
Cilic, who made 46 unforced errors, dropped to 1-9 across his four appearances at the tournament.
''The win, that's the most important thing,'' said Zverev, who failed to advance from the round-robin stage on his debut last year.