Djokovic beat Isner in the ATP opener
The opening matches at the ATP Finals led to a debate over whether conditions at the O2 Arena were trickier than in past years.
Novak Djokovic ended it.
The Wimbledon and U.S. Open champion made an impressive start to his campaign for a sixth title by dismissing John Isner 6-4, 6-3 on Monday.
Roger Federer, who lost on Sunday, and Alexander Zverev described conditions as challenging following their error-strewn opening matches at the season-ending tournament, but Djokovic dropped just six points on his serve and made six unforced errors.
"My results here have been pretty good in the last 10 years, so I can't complain too much," said Djokovic, who has won four titles at the London venue. "It takes a little bit of time really to get yourself adjusted to the surface because the surface is a good surface."
A day after a ceremony to mark ending the season as No. 1, Djokovic underlined his status by nullifying Isner's usually destructive serve.
Isner, who at 33 became the season-ending tournament's oldest debutant since 1972, applied himself admirably but barely avoided being broken more than once in the first set, and was broken twice more in the second set.
"It was amazing," Isner said of the atmosphere. "I wish I could have given them a bit more to cheer for."
The one break Djokovic converted in the first set, at 2-2, was a cross-court forehand return winner which stranded Isner.
Cristiano Ronaldo was in attendance, with Djokovic perhaps spurred on even more by the Juventus forward's presence.
Ronaldo was seen along with his partner Georgina and his son Cristiano Ronaldo Junior 
"It's great to have big football star, sports star like him, courtside watching tennis," Djokovic said.
At 3-3, 15-30 in the second set, Isner sent down a second serve at 126 mph (203 kph). Djokovic sent it straight back at his feet and a shocked Isner hit his backhand long to concede two more break points.
A forehand error from Isner confirmed the break, and the end of the contest. The merciless Djokovic held serve before breaking Isner once more with an inch-perfect backhand passing shot on match point.
"That was pretty comprehensive out there," Isner conceded.
Earlier, 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 attritional 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.
