The 20-time Grand Slam champion Roger Federer, who announced his retirement from professional tennis last year, was seen recently attending Coldplay's 'Music Of The Spheres World Tour' concert in Zurich. The former world number one even crooned to one of Coldplay's songs on stage.

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Coldplay, the hugely successful band based in England, held their concert at the Letzigrund stadium in Zurich.

As soon as their lead singer Chris Martin is about to start another song on stage, he announces to the crowd that he will be joined by none other than Roger Federer. The Swiss ace is then welcomed to a rapturous applause as Martin and the band alongside Federer sing to 'Don't Panic'.