Roger Federer’s unhappy season continued yesterday 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 still not enough to stop him from being on the receiving end of a 7-6 2-6 6-4 defeat against the powerful Argentine and his 220 kph serve.

To add to his woes, victory would have guaranteed Federer a place in the season-ending World Tour Finals but his hopes of qualifying for the tournament he has won a record six times will be decided at the upcoming Paris Masters.

After Del Potro took the first set, the momentum seemed to swing Federer’s way when he raced through the second set.

But the turning point came in the first game of the third set when Federer was broken after leading 40-15. Del Potro hit a stunning return on his way to break point and, although the Swiss hero 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 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 second-round defeat at Wimbledon to Sergiy Stakhovsky before losing to Tommy Robredo in the US Open fourth round.

In Istanbul, Serena Williams capped off a season to remember by capturing her fourth WTA Championship title with a 2-6 6-3 6-0 win over Li Na yesterday.

The 32-year-old finished off a dominant year, in which she also won the French and US Opens, by scooping up her 11th title of 2013.

China’s Li, whose run to the final helped her to climb to third in the world – thus becoming the highest-ranked Asian woman ever – broke Williams’ serve twice in the first set.

However, the world number one shifted gears in the second set to roar through 6-3.

Williams showed little signs of fatigue after playing more than 80 games this season, as she flattened Li in the third set with her power and on court coverage.