Del Potro sets up Djokovic showdown
Juan Martin del Potro will play world number one Novak Djokovic in the last four of the ATP World Tour Finals after beating defending champion Roger Federer.
The Argentine's 7-6 4-6 6-3 victory meant he and Federer qualified from Group B. The Swiss star will now face Andy Murray in the other semi-final on Sunday.
I put all my concentration when I was serving and that helped me to make fantastic first serves and many aces.Juan Martin del Potro
Del Potro is 9/1 to win the competition, while Djokovic can be backed at 4/6, Murray at 5/2 and Federer at 7/2.
In Saturday's other game, David Ferrer won the dead rubber against Janko Tipsarevic 4-6 6-3 6-1.
Del Potro said after his fine victory against Federer: "I didn't think about the semis or trying to qualify for the next round. I was just thinking about my match.
"In the end, I was thinking maybe this could be my last service game of the year if I lose against him. So I put all my concentration when I was serving and that helped me to make fantastic first serves and many aces."
Del Potro saved three break points in the eighth game of the first set before dominating the tie-break but Federer his back to win the second set after clinching an early break.
But 2009 US Open champion Del Potro found his top form in the decider, firing down 82 per cent of his first serves and claiming the crucial break in the second game.
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