"It's exciting. The idea was there six or nine months ago," Federer told a news conference at the Monte Carlo Masters last week.
"I said it would be fun to do something like this. It never happened before," added Federer, who has won the last four editions of Wimbledon.
"It might seem crazy, especially at this time of the season but we also enjoy playing tennis for fun.
"It does not always have to be serious. The result is not the most important, it's more about having fun out there."
Federer is the undisputable king of grass, while double French Open champion Nadal is on a record 71-match winning streak on the slow clay surface.
Last year, the Spaniard beat Federer in the final of the French Open only for the dominant Swiss to claim his fourth Wimbledon crown by taming Nadal at the All England Club.
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