By Martyn Herman (Reuters) – Roger Federer, regarded by many as the greatest male player to wield a tennis racket and who took the sport to new heights during a career spanning more than two decades, will retire after next week’s Laver Cup in London. The 41-year-old, who won 20 Grand Slam singles titles and re-defined a sport with his artistry and grace, broke the news tennis fans across the world have long dreaded in a lengthy statement on Thursday. A knee injury meant he has not played a match since losing at last year’s Wimbledon, although many still hoped he could come back for one last go…