Del Potro out to upset the odds
The semi-finals at the Barclays ATP World Tour Finals will feature "three big names and one big guy", according to Juan Martin Del Potro.
The Argentinian made the self-deprecating quip following his 7-6 (7/3) 4-6 6-3 victory against two-time defending champion Roger Federer at London's O2 Arena.
That completed Sunday's semi-final line-up, with Del Potro joining Federer, Novak Djokovic and Andy Murray, although the match-ups will not be decided until after the final Group B match between David Ferrer and Janko Tipsarevic tonight.
The pair are in the unusual position of having no chance to qualify but still having an impact on those who have, with a Ferrer win meaning it would be Murray against Federer and Djokovic against Del Potro while a Tipsarevic victory would reverse the fixtures.
Federer had beaten Del Potro in 13 of their previous 16 meetings but the Argentinian has often given him problems, particularly this year in tight matches at the French Open and the Olympics.
And Del Potro won their most recent match in the final of Federer's home-town tournament in Basle two weeks ago, making it back-to-back indoor titles after also winning in Vienna.
The 24-year-old is back at the O2 for the first time since reaching the final in 2009 having battled his way back from a serious wrist injury, but he was reluctant to talk up his chances of a repeat.
Del Potro said: "Beating Federer is not easy for sure. But I didn't think about trying to qualify for the next round.
"I was just thinking about my match and doing the same things like Basle. At the end I was thinking maybe it could be my last service game of the year.
"So I put all my concentration into serving and that helped me to make fantastic first serves and many aces.
"But he's still the favourite to win the tournament, with Murray and Djokovic. Now you have three big names in the semi-finals and one big guy.
"I'm the number four for sure. But everything can happen. I beat Federer, Novak and Andy a long time ago, so maybe if I am lucky I can repeat that here."
In a tight first set it was Federer who had the only chances to break, three of them in the eighth game that he could not take, and a poor tie-break from the world number two put him behind.
He hit back straight away by winning the first eight points of the second set, but he was broken at the start of the decider and won only two points against the Del Potro serve in the final set.
"It's disappointing," said Federer, who was already assured of a place in the last four. "I had a good feeling going into the match, definitely the right game plan.
"It's indoor tennis, he has a big serve and all those things. But still I expect myself to get more returns into play.
"But it's important to move on right now. Mentally, it's important to look ahead and rest because it's a quick turnaround.
"I tried everything I could and was a bit frustrated at times, but it shouldn't affect me tomorrow."
Federer does not believe playing two days in a row will be a disadvantage for him or Del Potro, but admits that could become a factor if either makes it through to Monday's final.
The 31-year-old, meanwhile, admitted a major source of disappointment was that his defeat eliminated world number five Ferrer from the competition before his final match.
The Spaniard has been one of the players of the season, winning seven titles, the best on the ATP World Tour, and reaching the semi-finals at the French Open and US Open.
Federer added: "I was thinking more about David than I was thinking about Andy and Novak, to be honest. I really wanted to give him a chance and give myself the best possible preparation for the semis. I really hoped I could win.
"It's not that I prefer David over Juan Martin, not at all, but I think he's had a stellar season.
"It was a given I was going to give my very best. But I'm more disappointed for him than I am about losing today."