Roger Federer has struggled a lot over the past two seasons. A severe right knee injury kept him in the pits for a long time, fueling the specter of retirement and preventing the King from playing when he wished. Suffice it to say that the former world number 1 played just 13 official matches last year, collecting nine wins and four losses.The round of 16 at Roland Garros and the quarterfinals at Wimbledon were his best results of the season, before a knee relapse forced him to undergo surgery for the third time in 18 months. The 20-time Grand Slam champion i …