Following Federer's victory over him in the Shanghai Masters final on Sunday, Rafael Nadal joked that the Swiss had to stop playing so well if he wanted to be invited back to his academy in Manacor.
The Spaniard, who was speaking to Federer during the pair's visit to Nadal's tennis academy, has lost to his long-term rival in all four of their encounters this season, including an epic five-set loss in the Australian Open final.
With Federer the only man standing in Nadal's way of ending 2017 as the world number one, he had a warning the 36-year-old.
"The invitation is always open for you," Nadal said, as per Forbes.
"The only thing that cannot happen again is that after coming here you play that well again.
"If that's going to happen, I don't invite you."
Although Federer laughed off Nadal's comments, the 19-time Grand Slam winner will be desperate to overtake his career-long rival in the rankings and finish 2017 as world number one, an achievement which would crown a remarkable year for the Wimbledon champion.
In Shanghai, Federer's serve proved too much for Nadal to handle, with the former racing to victory in just 73 minutes. The loss in Shanghai was Nadal’s first in sixteen matches, a run which included an imperious victory at the US Open in September.
For ten-time Roland Garros champion Nadal, Federer's form will be a major concern ahead of November's ATP World Tour Finals at London's O2 Arena.
The veteran duo have rolled back the years in 2017, sharing the four grand-slams - claiming two each to defy the younger generation of talent coming through.
If Nadal is to finish the year as world number one however, he will first have to overcome the knee injury which has ruled him out of this week’s Swiss Indoor event in Federer’s hometown Basel, an event won by Federer on seven previous occasions.