Switzerland's Roger Federer in action during his quarterfinal match against Canada's Milos Raonic at Wimbledon, London, on July 12. [Photo/Agencies]

After being eliminated by Raonic a year ago, Federer took the rest of the season off to allow his surgically repaired left knee to heal properly. He already had missed last year's French Open in May, ending a record streak of 65 consecutive appearances at Grand Slam tournaments, and then skipped the Rio de Janeiro Olympics and US Open, too.

When Federer returned in January, he was fit and full of energy, and won his 18th major championship at the Australian Open.

That was part of a 19-1, three-title start to 2017, before he took another break, sitting out the clay-court circuit and the French Open again.

"I could never really play quite so freely last year just because (now) I'm more focused on how the knee's behaving, rather than how I need to hit my forehand or backhand or what's not going to be good for my opponent," Federer said.

"This year, I'm just a normal tennis player again, where I can focus on tactics. I think that's the difference. I'm playing very well. I'm rested. I'm fresh. I'm confident, too. Then great things do happen."Such as becoming the oldest Wimbledon semifinalist since 1974.

Perhaps his rivals at the top of the game should take note.