Novak Djokovic made a superb opening campaign at the ATP Finals, defeating American John Isner in straight sets.
The World No.1 and favourite for a sixth London title was a comfortable 6-4, 6-3 winner over 6ft 10in Isner at the O2 Arena, London.
With Cristiano Ronaldo watching from the crowd, Djokovic secured his second break at 3-3 in the second and went on to dispatch Isner with the minimum of fuss.
Djokovic said: "It's great to be back in the 02, it's been a lucky place for me over the years.
"The match was great. I managed to get three breaks of John's serve which is sometimes mission impossible, but I managed to be at the right place at the right time, and I held my serve very well and played solid.
"I missed this tournament last year because of injury. I kind of enjoyed the time off but I missed this arena and I'm glad to be back."
Earlier, Alexander Zverev won two tiebreaks as he opened his ATP World Tour Finals campaign with a straight-sets win over Marin Cilic.
The German No. 3 seed, who also defeated Cilic in their opening match of last year's season-ending tournament, was down a break of serve in both sets but came through 7-6(5), 7-6(1) at the O2 Arena.
Croatian No. 5 seed Cilic now faces an uphill task to qualify from the group with two matches left to play.
Cilic entered the match with a dismal record at the O2 Arena -- having won only one of his previous nine matches across three appearances at the tournament -- but started strongly to break Zverev in only the second game of the opening set.
The Croatian had further chances to break at 3-0 and 4-1 but wasn't able to convert as Zverev dug in. With Cilic serving for the set at 5-3, Zverev broke back before holding serve with an ace to level at 5-5.
After both players held serve to force the tiebreak, Zverev took advantage of some wayward Cilic hitting to take a 5-2 lead, before converting his third set point with a passing backhand down the line.
The second set started quickly, with both players dominant on their serves. Zverev, who arrived in London having won three titles in 2018, had early chances to break at 1-0 and 3-2 but it was Cilic who edged in front after a weak backhand from the German handed his opponent the break.
Cilic's lead didn't last long, however, as an unforced error of his own gifted Zverev the break to bring it back to 4-4. Zverev then conjured up a match point with a sublime backhand winner at 30-30 at 5-4 but Cilic resisted and held serve with a strong serve down the middle. Zverev quickly held serve to put the pressure back on Cilic, who held on to force the second-set tiebreak.
Further errors from Cilic saw Zverev race to a 5-1 lead before a classy backhand winner and a unreturnable serve sealed the second-set tiebreak, as well as the opening win.
On the first day of play on Sunday, Kei Nishikori handed Roger Federer a surprise defeat as he won 7-6(4), 6-3 while Kevin Anderson made a winning debut at the tournament with a 6-3, 7-6(10) victory over Dominic Thiem.