Kei Nishikori beats David Ferrer 4-6 6-4 6-1
Ferrer replaces injured Raonic before match
Murray must beat Federer in straight sets for semi-final place
Federer confirmed into semi-finals
World Tour Finals at O2, London
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Don't forget you can watch this afternoon's match live on BBC Two, or listen to commentary on BBc Radio 5 live Sports Extra. Both are available by clicking on the Player at the top of this page.
Confused? I don't blame you. Suffice to say Andy Murray supporters should be cheering on Ferrer this afternoon in the hope that he can get one over on the US Open finalist from Japan.
Remember Nishikori beat Murray in the first round-robin match, so if the Scot and Nishikori were both to lose, the Japanese would go through on head-to-heads.
Raonic's withdrawal means Andy Murray will now have to beat Roger Federer tonight to have any chance of qualifying for the semi-finals. Ferrer could play the match of his life, but he will not be able to qualify for the last four. Here's the nice simple explanation of how things could pan out, courtesy of the ATP Tour.
Less than two hours before his scheduled meeting with Kei Nishikori at 14:00 GMT, world number eight Milos Raonic withdrew with a quad injury. Sadly, former Manchester United striker Ole Gunnar Solskjaer had other plans, so Spain's world number 10 David Ferrer steps in.
Image source, Getty ImagesGood afternoon tennis fans. Welcome back to our coverage of the ATP World Tour finals in London. I arrive with news of a last-minute Supersub....