Rafael Nadal made a winning return to the O2 Arena after two years away as he beat David Ferrer in his opening match at the ATP World Tour Finals.
The world number one began his bid for a first season-ending title with a 6-3 6-2 victory over his fellow Spaniard in one hour and 14 minutes.
Nadal, 27, joins Stanislas Wawrinka at the top of Group A, ahead of Ferrer and Tomas Berdych.
Novak Djokovic takes on Roger Federer in Group B at 20:00 GMT.
Nadal will secure the year-end number one ranking with one more win this week, an incredible achievement having only returned from a seven-month injury break in February.
The knee problem kept him away from the season finale last year, and his large fan base among the near 17,000 spectators was clearly thrilled to have him back.
"They are never easy games against David Ferrer, the number three in the world," Nadal told BBC Sport. "He is great and having an amazing career.
"He played a lot better than me in Paris last week but today I played a little bit better than him and it's important for my confidence to start with a victory after Paris.
"There is no rest [for tomorrow] but the energy of this stadium is fantastic and hopefully I am ready for tomorrow against Stanislas Wawrinka."
Ferrer had scored a surprise win over the 13-time Grand Slam champion in Paris only three days earlier, but there was no danger of a repeat performance in London.
The world number three admitted before the match that he was feeling the affects of his best-ever season, and the normally reliable baseliner made far more errors than usual.
Despite recovering an early break of serve, Ferrer's forehand was especially shaky, and he found the tram lines in game five to drop behind for a second time.
Nadal was in front without hitting anything like top gear, but he raced across the baseline to hook a brilliant forehand winner up the line on his way to claiming the set with a third break.
After little over half an hour, and with the match now firmly under his control, the French and US Open champion began to find his range.
Ferrer, 30, was barely winning one in four points behind his first serve as Nadal powered through the second set on a run of seven straight games.
There was a late rally from Ferrer, who got a break back when he caught out his opponent at the net, but a 19th forehand error of the afternoon gave Nadal victory on his second match point.
Asked what had made the difference so soon after losing to Ferrer in Paris, Nadal said: "Probably he arrived a little bit more tired after playing the final there. There was not a lot of time to adapt, the court is a little bit slower.
"I played with a little bit more calm than the other day. And, in my opinion, he didn't have the chance to hit the winners as easy as he did three days ago."