Roger Federer turned to a familiar face for inspiration ahead of this year's season: Rafael Nadal.

The Swiss was recovering from a knee injury that had forced him to undergo surgery for the first time in his 18-year career. The Spaniard, often troubled by knee problems, was out with a wrist injury. Many expected their longest Grand Slam droughts - five and three years respectively - to continue. But Nadal's past gave Federer belief amid their uncertain tennis futures.

"The way you came back from injuries, you made it seem easy, when you broke into the top 10, top five again and became world No. 1," the Swiss told his rival and friend when he helped to open the Rafa Nadal Academy in October last year.

"I hope to live something like this when I am back on the tour in January."

At the end of January, Federer was back in the top 10 after starting the month as No. 16. The greater prize was winning the Australian Open title, defeating Nadal in the final.

By October, the 36-year-old added six more titles for his biggest haul since winning eight in 2007. Among them was another Grand Slam crown - a record-extending 19th overall - at Wimbledon.

Like Federer at the All England Club, Nadal won the French Open without dropping a set. It was the 31-year-old's 10th Roland Garros crown after achieving "La Decima" in Monte Carlo and Barcelona in April. No other tennis player in the professional era has won 10 titles at the same tournament.

The King of Clay, without a hard-court title since 2014, then got two in a season, winning the US Open and China Open. By then, he had regained the world No. 1 spot, while Federer was just a rung below. The last time they had occupied the top two spots was March 20, 2011. In short, after 2,368 days, tennis had a familiar look about it again.