(CNN) -- Tennis' year-end championships are supposed to showcase the top eight players in the world but Monday's opening matches in London took a backseat to another butting of heads between the two biggest names in the sport. The difference of opinion came as Stanislas Wawrinka beat Tomas Berdych 6-3 6-7 6-3 in the afternoon and Juan Martin del Potro defeated Richard Gasquet 6-7 6-3 7-5 in the nightcap. Having disagreed about a potential two-year ranking system and a previous selection of the ATP's executive chairman, the latest dispute between Roger Federer and Rafael Nadal revolves around the surface for the World Tour Finals.
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Tennis legends disagree on nature of ATP World Tour Finals
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Roger Federer argues the case for an indoor tournament while Nadal wants an outdoor event
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2013 action got underway on Monday as Stanislas Wawrinka beat Tomas Berdych
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Juan Martin del Potro rallied to beat Richarg Gasquet in the evening session
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