(Adds late match)
LONDON Nov 6 (Reuters) - Roger Federer reached yet another career milestone with victory over Janko Tipsarevic and David Ferrer dipped into his seemingly endless tank of stamina to beat Juan Martin del Potro on the second day of the ATP World Tour Finals on Tuesday.
Swiss maestro Federer glided to a 6-3 6-1 victory over outclassed Serbian Janko Tipsarevic to open his Group B account, taking his haul of victories at the elite year-ending event to 40 having qualified for an 11th consecutive year.
Another silky display from the 17-times grand slam champion took him past the record he shared with Ivan Lendl, the level of his play suggesting the 31-year-old world number is still the man to beat at the tournament he has won a record six times.
Ferrer does not glide, rather hustles and bustles his way around the court, driving opponents to distraction with rock-like defences, punchy groundstrokes and speedy footwork.
Del Potro will testify to that after the towering Argentine lost a fascinating duel 6-3 3-6 6-4 in front of a packed audience at the 17,000 O2 Arena alongside the River Thames.
Fourth seed Ferrer, whose victory was the 73rd of a season that has delivered a career-best seven titles, will face Federer on Thursday trying to gain a first victory at the 14th attempt.
Hardly a cause for optimism in the Spanish camp but on current form few players, not even Federer, will relish a meeting with the player nicknamed the Iron Man.
At least the evening singles was a proper contest.
Federer's demolition of an out-of-sorts Tipsarevic was a non-event, lasting little more than an hour.
The world number two seized the initiative when he broke Tipsarevic's first service game and never looked back.
"I was striking the ball well today," world number two Federer told reporters.
"I hope it's a sign for more to come. I'm happy the first one went so well and it takes the pressure off looking ahead in the next couple of matches."
Tipsarevic, who pulled out of his match at the Paris Masters last week citing illness, qualified for the eight-man tournament as a result of Rafa Nadal's continuing injury woes.
He also went into the match with a zero in the wins column in previous clashes with Federer and there was an air of predictability about the result almost from the word go.
One break was enough for Federer in the opening set and after he pounced again to break for a 2-1 lead in the second he galloped away to victory, the 876th of his career to move into third place of all time ahead of John McEnroe.
Ferrer fought off a break point against the towering Del Potro, runner-up here in 2009, before moving 3-1 ahead when the former U.S. Open champion missed a backhand volley.
That proved enough for the Spaniard to take the opening set and he was still the dominant force at the start of the second set when he carved out four break points at 1-1, only for Del Potro to dig himself out of a gruelling service game.
The escape seemed to galvanise Del Potro who began clattering some heavy groundstrokes to gain a decisive break in the second set and set up a decider.
Ferrer stormed into a 4-1 lead only for Del Potro to drag himself right back into contention at 4-4.
Ferrer, who arrived on a 10-match winning streak after indoor titles in Valencia and Paris, refused to panic, however, and he rattled off the last eight points of the match to seal victory in two hours 16 minutes.
"I am very, very, very happy. It was a top match, and it was very difficult to play against Juan Martin," Ferrer said.
"I don't know the secret of my good form. It has been the best season of my career."
The focus returns to Group A on Wednesday when U.S. Open champion Andy Murray and world number one Novak Djokovic, who both won their opening matches, meet in a highly anticipated showdown. (Editing by Pritha Sarkar)