Federer hails Nadal's 'greatest achievement'
Nadal demolished Novak Djokovic 6-0, 6-2, 7-5 to add a 13th Roland Garros to his four US Opens, two Wimbledons and one Australian Open crown.
"I have always had the utmost respect for my friend Rafa as a person and a champion," tweeted Federer, who sat out the tournament to recover from knee surgery.
"As my greatest rival over many years, I believe we have pushed each other to become better players.
"Therefore it is a true honour for me to congratulate him on his 20th Grand Slam victory.
"It is especially amazing that he has won Roland Garros an incredible 13 times which is one of the greatest achievements in sport. I hope the 20 is just another step on the continuing journey for both of us."
Federer won his 20th and most recent Grand Slam title at the 2018 Australian Open and despite having already turned 39, the Swiss star still intends to return to action in 2021.
"I think he's happy when I'm winning and I'm happy when he's doing the things well," said Nadal.
"It means a lot the positive relationship that we have together because we have been going through a great rivalry for a very, very long time."