I think Roger Federer's win at the Australian Open 2017 was an incredible story. Even though he was an absolute legend of the game already, he had not won any grandslams in 5 years, continously losing to the same players and in general looking like it might not be enough ever again. Then he had his knee operation and most of the tennis world thought that this might be about it for Roger. Retirement beckons: 35 years old (which is extremely old in tennis), hasn't won anything really big in 5 years, injury problems, confidence problems.
Then he comes back playing a single fun tournament as preparation! Nobody gave him a shot at the tournament. He started with two laboured wins against mediocre opposition. Then the first test against Berdych, which he surprisingly passed with flying colours. Next up is Nishikori: young, fresh, full of confidence. Roger wins in 5 sets, but by now everyone agrees that the old dog can't play more than one five-setter in a tournament anymore. He dispatches Mischa Zverev easily, but then faces Stan Wawrinka in the semis. Stan had just won the US Open and was full of confidence, probably very close to the best tennis in his career. Roger again wins in a marathon five set match, and faces his old nemesis Nadal in the final. 3-1 down in the fifth set of the final, he plays some of the most magical tennis I have ever seen to win the Australian Open. If you ever watch a set of tennis, make sure it is that fifth set of the Australian Open Final 2017. Simply stunning, from two absolute legends. For me, this comeback is up there with the best sporting achievements in single player sports of all time.