( 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 should n't affect me tomorrow . '' Dad 's recovery gives Djokovic focus . 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 . '' Djokovic edges Murray at ATP finals . 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 did n'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 . ''
Juan Martin del Potro beats Roger Federer to claim semifinal place in ATP World Tour Finals . Argentine 's straight - set victory ends the hopes of Saturday 's world No . 5 David Ferrer . Ferrer finishes on a high note as he beats Janko Tipsarevic in final round - robin match . Federer will face Andy Murray in Sunday 's semifinals , while Del ##gado plays Novak Djokovic .