“Everything is possible (for Federer),” Asia's top player predicted.
Federer is due for a return to the dirt after two seasons completely off the surface in three weeks at Madrid, his lone scheduled tune-up prior to the French Open, which he last played in 2015.
“It (winning Roland Garros) might be tougher for him than winning the Australian Open or Wimbledon, but I'm sure he can do anything if he tries 100 per cent,” said Nishikori.
“He's a lot different than anyone else on the Tour,” added the world number three who lost the final here a year ago as Rafael Nadal won his 11th title in the principality.
“I'm sure Roger can do very well on clay. He's still my idol, he's the best player in history.
“I'm happy to see him playing more tournaments, I hope I can play him again,” said the player who has beaten the 37-year-old Swiss three times and most recently last November at the year-end ATP Finals.
“He came from injury and won the Australian Open (2017). I'm sure he works harder than anyone else off court. I hope he does well,” NIshikori said.