PARIS—Admit it: You aren't surprised that Roger Federer lost in the fourth round of the French Open on Sunday, 6-7(5), 7-6(3), 6-2, 4-6, 6-3 to Ernests Gulbis.

It's 2014, and tennis has changed. Federer hasn't been a sure thing at Grand Slams for years. He says he feels better than he has in a while, and certainly much better than last year, when he played with back pain. But a 32-year-old man with four children—two sets of twins, the second set born in May—can only feel so good on a tennis court, and it's tough to predict...