(CNN) -- Roger Federer has vowed to bounce back from his second successive defeat against a man who has gained a recent edge over the 17-time grand slam champion.
Juan Martin del Potro made his name by beating Federer in the 2009 U.S. Open final, and won again at that year's ATP World Tour Finals, but lost seven in a row before toppling the then world No. 1 in the final of his home event in Basel last month.
The Argentine, who towers over most opponents at six foot six inches tall, followed that up on Saturday with a 7-6 (7-3) 4-6 6-3 victory to qualify for the semifinals of the season-ending showpiece in London.
"It's disappointing," said defending champion Federer, who was already assured of a place in Sunday's semifinals after beating David Ferrer on Thursday.
"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."
By losing the first set, Federer ended the hopes of 2007 runner-up Ferrer -- who beat Janko Tipsarevic 4-6 6-3 6-1in the final match of the round-robin stage on Saturday.
Ferrer's victory means that Federer faces world No. 3 Andy Murray on Sunday in a rematch of the Wimbledon and Olympic finals, while Del Potro plays No. 1 Djokovic -- who won all three of his Group A games.
World No. 5 Ferrer is one of the ATP Tour's in-form players this season, with a leading seven titles -- including his last two outings in Valencia and Paris, where Federer relinquished his title and the No. 1 ranking when he decided to skip the season's final Masters event.
The Spaniard now has a tour-leading 74 match wins this year, one more than Djokovic.
"I really wanted to give him a chance and give myself the best possible preparation for the semis. I really hoped I could win," said Federer, who is seeking a record-extending seventh title at the elite eight-man event.
"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."
Del Potro is appearing at the ATP World Tour Finals for the first time since 2009, when he lost in the final in the tournament's debut year in the UK capital.
"Beating Federer is not easy for sure. But I didn't think about trying to qualify for the next round," said the 24-year-old, who has battled his way back into the top-10 after long-term wrist problems.
"I was just thinking about my match and doing the same things like Basel. 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 favorite to win the tournament, with Murray and Djokovic.
"Now you have three big names in the semifinals 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."