He is the only player to hold titles on all four court surfaces.

他是唯一一个在四种场地上都获得冠军头衔的选手。

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The maths of the clay surface are distinctly different to grass or hard-court surfaces.

CNN: Tennis interactive: Why clay is so different

When he returns he will be on his favourite hard court surfaces and he understands what he needs to do.

BBC: Wimbledon 2011: Tim Henman on Andy Murray

The 27-year-old, who injured his left knee last October, is hoping to avoid more knee problems by cutting down on the number of times he changes court surfaces.

CNN: Federer crashes to Davydenko in semis