A piece of tennis history is up for auction -- the shirt worn by Rafael Nadal en route to his first-ever Grand Slam title at the 2005 French Open ... and it's expected to fetch a pretty penny!!
TMZ Sports is told the match-worn shirt -- which Nadal wore during the quarterfinal, semifinal, and final at Roland Garros -- is being made available via Prestige Memorabilia.
We're told the legendary green sleeveless tee had a starting bid of $25,000 .