MELBOURNE, Australia — There were tears, but that was to be expected from Roger Federer at the Australian Open. In the end, the most remarkable part of his 20th Grand Slam singles title was that it came as no surprise. Not even at age 36 in a sport where the spoils have generally been reserved for a much younger crowd. Federer, like Serena Williams, has redefined the limits. After going nearly five years without a major title, he has now won three of the last five in a phase of his career when he insists that he would have been content with just one more. “I’ve won three Slams in 12 months,” he said. “I can’t believe it myself.” On Sunday, under a closed roof in Rod Laver Arena, Federer secured his sixth Australian Open by recovering his mojo in time to hold off Marin Cilic, 6-2, 6-7 (5), 6-3, 3-6, 6-1. Federer is the oldest man to win the Open since one of his role models, Ken Rosewall, in 1972. It was played on grass then in the much more intimate setting of the Kooyong Lawn Tennis Club , with no retractable roofs in sight. Though seeded No. 2…