World number two Federer, the favorite after the shock defeat of four-time winner Rafael Nadal, as well as the exits of Andy Murray and Novak Djokovic, will face Argentine fifth-seed Juan Martin del Potro, who put out Tommy Robredo, for a place in what would be a fourth successive Roland Garros final.
A victory in Sunday´s title match would make the Swiss only the sixth man in history to complete a career Grand Slam and take him level with Pete Sampras´s record of 14 majors.
Defeat for Monfils ended, for another year, hopes of a first French men´s champion at Roland Garros since Yannick Noah in 1983.
"It was very important to win the first set. Gael felt it mentally and physically," said Federer.
"I tried to play aggressively and because of that I made a lot of errors. Perhaps I took too many risks."
Federer will approach the rest of his Roland Garros mission with confidence due to his impressive records against the remaining men in the draw.
Fernando Gonzalez, who tackles Robin Soderling in the other semi-final, has one win in 13 matches against the Swiss star while Soderling is 0-9 and Del Potro, 0-5.
Monfils, whose subdued, on-court demeanor was in stark contrast to to his fist-pumping, chest-beating dismissal of Andy Roddick in the last round, admitted the match got tougher as it went on.
"I had lots of chances in the first set but I couldn´t make the most of them. I was a little disappointed by that and after it got a lot harder and he played well," said the 11th seed.
"I was totally off in the second set and this was the big disappointment because if I want to be aming the very top players I have to focus more and fight."
In a tense opening set, Federer demonstrated the pressure he is under to finally deliver a French Open title when he engaged in an uncharacteristic and lengthy dispute with chair umpire Mohamed Lahyani over a line call.
He then saved a set point in the tie-break before clinching the opener with a crisp forehand volley, a huge relief for a man who was forced to battle back from two sets down to beat Tommy Haas in the last 16.
Monfils hadn´t played for a month in the run-up to Roland Garros because of a badly-inflamed knee, and having seen his first set opportunity slip from his hands, he was quickly two breaks down to trail 4-0 in the second.
The, who had been defeated by Federer in the quarter-finals here last year, stopped the rot with a service hold in the fifth game.
But Federer wrapped up the set when Monfils buried a weary forehand return into the net as the steamy Paris afternoon began to take its toll.
Monfils twice called the trainer onto court in the third set, but Federer wasn´t to be denied. The Swiss broke to lead 5-4 when Monfils ran round on a forehand, only to drive the ball into the net.
Federer then went to three match points and took the tie with a smooth, down-the-line backhand winner.
Earlier, Argentine fifth seed Juan Martin del Potro reached his first Grand Slam semi-final on Wednesday when he defeated Spain´s Tommy Robredo 6-3, 6-4, 6-2 in the French Open quarter-finals.
The Argentine had beaten Robredo, who was playing in his fifth Roland Garros quarter-final, on the only previous occasion the two had met in Madrid in 2007.
The 20-year-old was quickly into his stride on Wednesday breaking the Spaniard in the seventh game on his way to claiming the first set after 34 minutes.
He eased through the rest of the match although he needed five match points to wrap up the tie.
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