Roger Federer blazed past Fernando Verdasco after an early stumble on Sunday while Andy Murray marked the start of London's five-year hosting of the ATP Tour Finals with a three-set victory over Juan Martin del Potro.
Swiss maestro Federer, looking to regain the year-end top ranking after being overthrown by Rafael Nadal last year, dug himself out of trouble to win 4-6 7-5 6-1 and join Murray at the top of Group A in the season-ending showpiece.
World number one Federer was top billing for the evening session at the spectacular 02 Arena but lost his opening service game to love and sprayed 15 errors in the first set as Verdasco threatened to cause a shock.
He then survived a crisis at 5-5, 0-30 in the second set, breathing a sigh of relief as the left-handed Verdasco went agonisingly wide with a forehand -- a near-miss confirmed by a Hawkeye replay accompanied by thumping heartbeat sound effect, which was just part of the razzmatazz enjoyed by the 17,500 crowd.
Crisis over, 15-times grand slam champion Federer broke serve for the first time in the match in the following game to take the set and then raced away to victory.
"I was down a set, and only the second set was I able to sort of get the ball into play, find my range, find my rhythm," Federer, a four-times winner of the tournament, told reporters.
"The longer the match went, the more my belief was going up and his was going down."
Six-times Wimbledon champion Federer said he was impressed with the venue and the lighting, which leaves the crowd in virtual darkness as the players duel on a vivid blue court.
"I thought the crowds were great. The lighting is unique. We don't really see the crowd and the focus is very much on us. It's a bit unusual," he said. "It's not Wimbledon like, but still special obviously."
Murray withstood a ferocious comeback by U.S. Open champion Del Potro to claim a 6-3 3-6 6-2 victory that at one stage looked to be slipping away from him.
Del Potro, who lost the first five games, scorched some huge forehand winners but was finally undone by the wily world number four, who soaked up everything the burly Argentine threw at him.
"He didn't start particularly well but after 5-0 I thought the standard was very good," Murray, who has won more titles this season than any other player on Tour, told reporters.
"He's got a big serve, long reach and goes for huge shots. You just have to try and get through it. Tactically I've always been fairly good so I found a way through today."
London's O2 Arena, formerly known as the Millennium Dome, is more used to welcoming the world's greatest pop stars, and there was plenty of glitz as the $5 million event got under way with hardly an empty seat in sight.
Led Zeppelin blared out of the sound system and spotlights punctured the eerily-lit arena as Murray and Del Potro walked out to begin the sixth instalment of what many predict will become one of the fiercest rivalries in men's tennis.
Del Potro, a year younger than the 22-year-old Murray but already one-up on the Scot in the grand slam stakes after his stunning victory over Federer in the Flushing Meadows final in September, arrived at the venue by boat and looked all at sea in the first set in which he was troubled by a nose bleed.
"I have a big nose, that's the problem," a glum Del Potro told reporters, although some of his play from 0-5 in the first set should give him confidence that he can still progress.
Things began to change as Del Potro found his timing to claw it back to 3-5 and, although Murray finally clinched the opener on his seventh set point, the tide had turned.
Del Potro broke serve twice in the second set and after softening up Murray with some punishing inside out forehands that Murray could merely slice back defensively he levelled the match with an unstoppable crosscourt effort.
As quickly as it had sparked into life, however, Del Potro's challenge withered and world number four Murray won the third set surprisingly comfortably after breaking in the second game.
"I could win two matches and still not get to the semi-finals," said Murray. "But winning the first one obviously helps a lot."
Group B action opens on Monday when Rafael Nadal, who can still beat Federer to the year-end world number one ranking, faces Swede Robin Soderling and Novak Djokovic takes on Nikolay Davydenko.