ATP World Tour Finals
Venue: O2 Arena, London Date: 22-29 November
Tournament coverage: Live on BBC Two, 28/29 November, 1430 GMT; every match live on BBC Radio 5 live/5 live sports extra; live text commentaries on BBC Sport website; live coverage every day on Sky Sports
Federer had never lost a set to Verdasco before tonight's match
By Piers Newbery
BBC Sport at the O2 Arena
World number one Roger Federer recovered from a poor start to beat Spain's Fernando Verdasco in the first evening session of the ATP World Tour Finals at London's O2 Arena.
Federer was broken to love in the opening game and did not fashion a break point until game 12 of the second set.
But he converted that opportunity to level the match and eased through to win the round-robin clash 4-6 7-5 6-1 and take his career record against Verdasco to 4-0.
Andy Murray beat Juan Martin del Potro in Sunday's first singles match, and he and Federer will meet on Tuesday as the early favourites to take the two semi-finals places from Group A.
Verdasco will take on Del Potro with both men knowing a win is vital if they are to maintain a realistic challenge.
The roar for his arrival showed the London crowd was thrilled to see six-time Wimbledon champion Federer back in London for the second time in 2009, but the top seed made a dreadful start.
Federer opened with a double-fault and followed up with two forehand errors and a backhand into the net as he dropped serve to love.
Verdasco has long had a reputation for the spectacular but this year has managed to harness his talent on a more consistent basis, hence his first appearance at the season finale.
The Spaniard made a major breakthrough when he served Murray off the court at the Australian Open in January before losing a five-hour epic against Rafael Nadal in the semi-finals.
And it was the big serve and heavy forehand combination that had Federer in trouble early on Sunday evening as the Swiss struggled to keep the error count down, at one stage swiping his racquet in frustration at the court.
Verdasco failed to take advantage of two points for the double break in game seven but he was rarely troubled on his own serve and capitalised on the early break by serving out the set.
Federer continued to make little impact as the second set developed, a fizzing Verdasco cross-court backhand flying past him as he approached the net at 30-30 in game six before the eighth seed roared from 0-30 back with some magnificent play in game eight.
World number eight Verdasco looked on course to spring a major suprise
A pumped up Verdasco could sense victory when he moved 0-30 clear at 5-5 but Federer held on, and moments later the match had turned on its head.
The showboating side of Verdasco reappeared at the worst possible moment as he mishit an overhead from the baseline so badly that it hit the umpire's chair and Federer converted his first break point of the day to level the match.
Federer was in charge now and three beautiful backhand winners helped him break at the start of the decider as Verdasco's confidence drained away.
The French Open and Wimbledon champion, looking to join Ivan Lendl and Pete Sampras as the only men to win the season-ending championships five times, powered through the set to win in one hour and 59 minutes.
"I felt at the beginning of the second set I was starting to return better," said Federer. "At the beginning the first game was so quick, four points, I was running after the score because of that and the whole first set I was struggling to put returns into play."
Federer praised the new venue after his first experience.
"The crowds are great, the lighting is unique - you do see the crowd but the focus is very much on us. This is a bit unusual but we had the chance to practice in these surroundings and I thought it was very nice to play in. It's not like Wimbledon but it's still special."
He added: "I thought the playing surface was OK, there were some bad bounces but that always happens on these wooden floors we play on. But it felt like a proper court, fair for all playing styles - if you want to play far back you can do that, if you want to move in you can do that too."
Asked afterwards what happened to so dramatically change the match at the end of the second set, Verdasco said: "He was playing good and had his chances.
"I also had my chances at 5-5, 0-30 - tennis is like that. After that my fitness was going down and he also got confidence, and the match changed. It was not nerves."
In Sunday evening's doubles match, third seeds Mahesh Bhupathi and Mark Knowles defeated fifth seeds Frantisek Cermak and Michal Mertinak 6-3 6-3.
Rafael Nadal, Robin Soderling, Novak Djokovic and Nikolay Davydenko kick off Group B on Monday.