Monfils, who was unseeded for the tournament, is the only player outside the world's top three left in the draw, with Novak Djokovic and Rafael Nadal contesting the other semi.
Federer saw off the fiery Chilean Fernando Gonzalez yesterday (Wednesday, June 4th), going through 2-6, 6-2, 6-3, 6-4 in a match lasting a touch over two hours.
Monfils, who took the junior title at Roland Garos in 2004, said: "I ran well on court and I wanted to take the ball early. I played great tennis today.
"I prepared really well. I tried to stay concentrated and with my family and the crowd behind me, I felt very strong."
Monfils beat Spain's David Ferrer 6-3, 3-6, 6-3, 6-1, to win his quarter-final on the Court Chatrier.
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