Federer 4-9 for O2 success after Nadal withdrawal
Carreno-Busta comes in as reserve for world number one
Continues 2pm Tuesday on BBC2 & Sky Sports Arena
Roger Federer is 4-9 to win the ATP Tour Finals for a seventh time after world number one Rafal Nadal withdrew from the annual finale at the O2 Arena after aggravating a knee injury in Monday night's three-set group-stage defeat to David Goffin.
Nadal had withdrawn from the Paris Masters after two rounds at the start of the month in order to try and be fit for a bid to lift the London title for a first time but he was clearly in pain throughout a punishing 7-6 6-7 6-4 battle.
Pablo Carreno-Busta now comes into the Pete Sampras Group as first alternate and will play Nadal's remaining matches against Dominic Thiem and Grigor Dimitrov.
Dimitrov has become a 6-1 joint-second-favourite for the event with Alexander Zverev after beating Thiem 6-3 5-7 7-5 in the Monday afternoon session.
The Boris Becker group continues on Tuesday afternoon with Marin Cilic against Jack Sock, who both lost their Sunday encounters with Zverev and Federer. Zverev edged out Cilic 6-4 3-6 6-4 while Federer held Sock at bay 6-4 7-6.
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