If Roger Federer loses his Australian Open semifinal match to Novak Djokovic on Thursday, there will likely be a simple explanation. He is out of gas. Federer, 38 years old, has needed to play more tennis to get to Thursday’s semifinal than in any of his previous 20 Grand Slam championship runs. So far in this tournament, his matches have averaged nearly two hours and 32 minutes long.

Source: Wall Street Journal January 29, 2020 15:54 UTC