"Watching you attempt to win it for the tenth time is not only remarkable ... it is inspiring.
"You make me believe in life that anything is achievable and nothing is impossible.
"Go get 'em."
The letter will be posted at the Nike store in Paris along with hundreds of other tributes to the Spaniard as he tries to improve on his stunning record at the French Open.
Agassi won the French Open in 1999 when he defeated Ukrainian Andrei Medvedev, coming back from two sets down to win a five-set thriller.
His impressive record of 51-16 at the event is blown out of the water by Nadal's feats on the same clay courts.
The 29-year-old has a stunning 70-2 record at Roland Garros, since his first appearance at the grand slam in 2005.
He is on track to improve on that record after making just three unforced errors in his 6-1 6-1 6-2 triumph over Groth.
He needed just 80 minutes to move into the second round against Facundo Bagnis.
"He wasn't in a mood to give me too many free points, I tell ya," Groth said.
"But you've got to expect that if you play him here.
"Playing Rafael Nadal at the French Open and you go through the introduction on court and they talk about all the years he's won it.
"His record here speaks for itself."
Asked what Nadal had said when the pair shook hands at the end of the match, Groth said: "He said, 'Good luck for the grass.'
"He's a nice guy, a class act and a quality player.
"He's actually having one of the best claycourt seasons he's had for a while"
