US Open 2013: Roger Federer admits to lack of confidence as defeat threatens London finale

Organisers of the Barclays ATP World Tour Finals might have to start re-thinking their publicity plans for the season-ending finale at the O2 Arena in London. Roger Federer, who has been the tournament's biggest attraction after winning it six times and playing in it for the past 11 years in a row, is in danger of failing to qualify for the elite eight-man field after suffering the latest defeat in what has been a nightmare year for the 17-times Grand Slam champion.

Federer's crushing 7-6, 6-3, 6-4 defeat by Tommy Robredo here in the fourth round of the US Open means that he has not reached the final of a Grand Slam tournament for the first year since he won his maiden Wimbledon title in 2003.

The field for the World Tour Finals comprises the eight players who have earned the most ranking points in the calendar year. Federer is currently seventh in the list – behind Rafael Nadal, Novak Djokovic, Andy Murray, David Ferrer, Juan Martin del Potro and Tomas Berdych – and has the likes of Stanislas Wawrinka and Richard Gasquet breathing down his neck.

When asked about qualifying for the year-end finale Federer said: "I'm not really thinking about it. I don't think I'll change my schedule accordingly. I believe if I do play well, I'll make good results. If I make it or not [to London], then we'll see. But what's important for me, first and foremost, is that I move better, that I play better."

Federer, who converted only two out of 16 break points, said he had "self-destructed" in losing to the world No 22, who had more trouble finding a way past Britain's Dan Evans in the previous round. "I kind of feel like I beat myself, without taking any credit away from Tommy," Federer said. "I just couldn't do it. It was a frustrating performance."

He added: "I struggled throughout, which is not very satisfying, to be honest. Tommy did a good job to keep the ball in play and make it difficult for me, but I missed so many opportunities. My rhythm was off."

Federer admitted that he was low on confidence, a shocking admission for a player who has always seemed so self-assured. "Confidence takes care of all the things you don't usually think about," he said. "It's been a difficult last three months. Maybe my consistency is just not quite there yet. Maybe on a daily basis, on a set by set or point by point basis, maybe that's something that has been difficult for me."

The Swiss had been on course for a quarter-final showdown with Nadal, but the world No 2, who beat Philipp Kohlschreiber in four sets, will now take on Robredo. Gasquet, who will face Ferrer, is the only non-Spaniard left in the bottom half of the draw. The Frenchman progressed by winning a five-set encounter with Milos Raonic lasting four hours and 40 minutes.

Mikhail Youzhny was the first into the quarter-finals from the top half of the draw thanks to a 6-3, 3-6, 6-7, 6-4, 7-5 victory over Lleyton Hewitt. Youzhny, who has twice reached the semi-finals here, recovered from 5-2 down in the decider to win after three hours and 58 minutes.

Such marathon matches have been used by some critics to back their argument that women do not deserve equal prize money because they play shorter matches. Andy Murray, who was facing Denis Istomin in the fourth round, told The New York Times that he thinks women should play over five sets rather than three, or that men should play three sets. "I think that's more what the guys tend to complain about, rather than the equal prize money itself," Murray said.

Victoria Azarenka, the world No 2, needed more than two hours to book her place in the quarter-finals, beating Ana Ivanovic 4-6, 6-3, 6-4. Azarenka will next meet Slovakia's Daniela Hantuchova.