Roger Federer will embark on his 21st professional season when he leads Switzerland in the mixed teams Hopman Cup starting in Perth, Australia tomorrow.The 36-year-old Swiss is coming off an extraordinary year where he took his grand slam tally to 19, with a fifth Australian Open title and a record eighth Wimbledon. Federer finished the year at number two in the world behind Spanish rival Rafael Nadal.Federer will again partner Belinda Bencic at the Hopman Cup, his only event before launching his Australian Open title defence on January 15.