(Corrects spelling in paragraph nine)
By Brian Homewood
BASEL, Oct 27 (Reuters) - Roger Federer's difficult season continued on Sunday when he lost to Juan Martin del Potro in the final of the Basel Open for the second year running.
The Swiss played some of his best tennis this year but it was not enough to avoid a 7-6 (3) 2-6 6-4 defeat against the powerful Argentine and his 220-kph serve.
After Del Potro took the first set, the momentum seemed to swing Federer's way when he raced through the second after breaking serve in the second game.
But the turning point came in the first game of the third set when Federer, who has managed only one win against a top-10 opponent and one tournament win all year, was broken after leading 40-15.
Del Potro hit a stunning return on his way to break point and, although the Swiss saved one point with an amazing backhand overhead smash, Del Potro took the game when Federer overhit a forehand.
Del Potro went on to take the set and win his fourth ATP title of the season after Tokyo, Washington and Rotterdam.
Federer would have qualified for the season-ending World Tour Finals if he had won, although he should still book his place there at next week's Paris Masters.
A win over Kevin Anderson or Mikhail Youzhny in his second round match on Wednesday will be enough to ensure he reaches the season finale for a record-equalling 12th consecutive year, tying with Ivan Lendl for most successive appearances.
Federer, whose season has been peppered with embarrassing defeats to lower-ranked opponents, was happy with Sunday's performance, where he interspersed some stunning winners with unforced errors including five double faults.
Yet he could not take comfort from the result.
"I am a winner, I don't take much confidence from losing matches," he told reporters. "I am happy about the level of my play, it's getting better but it's not in losing where I get much confidence."
Del Potro admitted he lost his way in the second set before recovering.
"I made a lot of mistakes and you can't afford to relax in a final," he said.
"My big challenge is to get closer to the top four players and the only way to do that is when you have a chance to play against them."
Federer, who won his last grand slam title at Wimbledon in 2012 before claiming a silver medal at the London Olympics, has failed to reach the final of a major tournament this season for the first time since 2002.
His only tournament success was at the low-key Halle event and he was beaten in the final at Rome.
He lost in the Australian Open semi-finals to Andy Murray, fell at the last-eight stage to Jo-Wilfried Tsonga in the French Open, suffered a shock second-round defeat at Wimbledon to Sergiy Stakhovsky before losing to Tommy Robredo in the U.S. Open fourth round.
His win over Tsonga in the Australian Open quarter-finals was his only victory against a top-ten ranked opponent this year. (Editing by Toby Davis)