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Andy Murray faces a straight shoot-out with Stan Wawrinka for a place in the semi-finals of the ATP World Tour Finals in London.
The tournament has thrown up all sorts of complex scenarios in the past but this time it could not be more simple.
Rafael Nadal has already guaranteed top spot in the group following his 6-4 6-1 over Murray and second place will be decided by the Scot's clash with Wawrinka on Friday evening.
The Swiss had been heavily criticised for his performance against Nadal on Monday but he showed his fighting spirit to recover from 2-5 in the opening set against David Ferrer and win 7-5 6-2.
Murray's performances at the O2 Arena have been a mixture of the good, the bad and the ugly, and he is bidding to reach the last four for only the third time since the event moved to London in 2009
He began well, breaking Nadal in the opening game. It was a poor game from the Spaniard but he quickly recovered and showed his growing confidence is well placed.
Nadal will finish the season without a grand slam title for the first time in a decade but he has been looking much more like his old self in recent weeks and began to turn the screw on Murray.
Nadal was playing aggressively, dictating with his forehand and keeping Murray firmly on the back foot.
The Scot fought off break points in the sixth and eighth games but his defences were breached as Nadal broke to love to take the opening set, and two successive double faults from Murray saw him broken again at the start of the second.
Murray only made 43 per cent of first serves and won just 34 per cent of second-serve points.
The 28-year-old, who bizarrely cut his fringe during the changeover after the third game, was impressed by Nadal's form but conceded he had not helped himself.
"He obviously played better than he did when I played him earlier in the year," said Murray, who produced one of the most eye-catching results of his career by beating Nadal on clay in the final of the Madrid Masters.
"Also I didn't help myself out there. I served extremely low percentage, maybe lowest percentage I served the whole year in any match. That's not good enough against someone as good as Rafa."
This was the first time Nadal had beaten a player ranked in the top two since the French Open last year, and all his rivals will be wary of the 14-time grand slam champion on this form.
Roger Federer, who was the first singles player to qualify, takes on Kei Nishikori in the afternoon match on Thursday while Novak Djokovic will look to bounce back from his loss to Federer and book his place in the semi-finals with victory over Tomas Berdych in the evening.
Also in action on Thursday evening are Jamie Murray and John Peers, who need to beat the top-ranked Bryan brothers to progress.