Story highlights Roger Federer tumbles in year-end rankings

Andy Murray, Angelique Kerber new year-end No. 1s

Maria Sharapova drops out of rankings

(CNN) Roger Federer, Maria Sharapova, Andy Murray and Angelique Kerber -- among others -- all played their part in a tennis season of firsts. It won't soon be forgotten -- or duplicated, and here's why...

Federer's struggles

Federer's season began in promising fashion, having reached the semifinals at January's Australian Open. But it went downhill from there for the much-loved Swiss record-breaker.

The 17-time grand slam winner injured his knee while running a bath for his twin daughters, necessitating surgery for the first time in his career. He subsequently missed the French Open with a back complaint -- the first major he bypassed since 1999 -- and shut down his season after re-injuring the knee at Wimbledon in July. At No. 16, the 35-year-old finishes 2016 outside the top 10 for the first time since 2001.