After a year that saw some uncharacteristic slip-ups from Federer in big matches, the Swiss star shrugged off a second-set letdown to beat Tsonga 6-3, 6-7 (6), 6-3 for a record sixth title in the ATP World Tour Finals.
"I know it's one of my greatest accomplishments," Federer said. "This definitely is an amazing finish again to the season. I've never finished so strong."
Federer was by far the most impressive player this whole week at the season-ending event for the top eight players in the world, where his three group-stage victories included his most comprehensive win ever over Rafael Nadal, beating the Spaniard 6-3, 6-0 on Tuesday.
But he looked in danger of faltering against Tsonga when he failed to serve out the match at 5-4 in the second set and then wasted a match point in the tiebreaker.
But Tsonga, who became the first player to rally from a two-set deficit to beat Federer in a Grand Slam tournament in this year's Wimbledon quarterfinals, couldn't pull off another surprise this time.
Playing in his 100th career final, Federer broke for a 5-3 lead in the decider and served out the match to love, sealing his 70th career title with an easy volley.
"I think that was the difference to other matches this year, where I lost so closely," Federer said. "This time around I was still able to find a way through. I think that was the difference at times during the year, which didn't maybe make this an absolutely phenomenal year."
Federer finished the season without a Grand Slam title for the first time since 2002, after also wasting a two-set lead against Novak Djokovic in the U.S. Open semifinals -- where the top-ranked Serb saved two match points in the fifth set.
Federer took a six-week break shortly after that -- in part, he said, to sort out his head after those losses to Tsonga and Djokovic.
"I feel when it happens maybe that often, I do have to question myself that maybe I did something wrong," Federer said. "I don't want to say underestimate or overestimate the mental part of the game, but there is a lot of time that goes by out on tour, during a match, you're just trying to stay positive. But you can't always be positive out there. You know, it's just too difficult. That's where maybe the doubts were just a bit too strong during certain important moments."
The break seems to have worked, as Federer is undefeated since that U.S. Open loss, and finished the year with a 17-match winning streak following titles in Basel and Paris before coming to London.
While Nadal and Djokovic both lost twice to be eliminated in the group stage, Federer played some of his best tennis of the year and beat David Ferrer in the semifinals without facing a break point.
"He's the best player indoors for the moment," Tsonga said. "He's maybe the best player ever, because he's really quick. He's playing well. That's it."
The capacity crowd for the final included Pippa Middleton, Cristiano Ronaldo and London mayor Boris Johnson -- who was initially cheered when showed on the big screen but received boos when he and his party were late to return to their seats after a changeover, holding up play as Tsonga was about to serve at 2-1 in the third set.
Tsonga said he wasn't bothered by the interruption.
"It happens all the time," he said. "That's no problem."
It was the third Sunday in a row that featured a meeting between these two players, as Federer beat Tsonga in the Paris final two weeks ago and again in their first round-robin match in London last weekend.
The third win proved the hardest for Federer.
The Swiss star looked headed for a fairly routine victory after breaking for a 3-2 lead in the second set, hitting a forehand return winner on the line. But he faltered when serving at 5-4, going down 0-40 before Tsonga converted his third breakpoint with a volley winner.
In the tiebreaker, Federer led 5-2 and then held a match point at 6-5, but Tsonga saved it with a forehand winner before ripping a scorching forehand return on his first set point to level the match at 1-1.
Federer's victory means he surpassed Ivan Lendl and Pete Sampras, who both won five titles at the season-ending event, previously known as the Masters Cup.
"I still don't feel like I'm better than Pete Sampras, or Lendl for that matter," Federer said. "I still believe they are (two) of the all time greats to play the game. I'm just happy to be compared to them."