The top seeded American has yet to drop a set at this year's event as he bids to capture the title for a sixth time.
Agassi has now won nine straight matches and took control of the first set after finally breaking Mathieu in the ninth game with the score tied 4-4.
He broke his opponent again in the opening game of the second set and held serve the rest of the way to close out the match.
Agassi's biggest threat comes from second seed Lleyton Hewitt of Australia, who skipped the Athens Olympics to prepare for next month's U.S. Open on American hard courts.
Hewitt advanced to the semi-finals earlier in the day, but had a much tougher time in his match before overcoming another Frenchman, Cyril Saulnier, 6-3, 3-6, 7-6.
The pair battled in scorching temperatures and the Australian needed to draw on all his reserves to overcome his unheralded opponent.
"He really picked up his game," Hewitt said.
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On Saturday, Hewitt will face fourth-seeded Robby Ginepri, who ousted Raemon Sluiter 6-3, 7-6, for a place in the final.
The fourth quarter-final between Luxembourg's Gilles Muller and Swiss qualifier Michel Kratochvil was a hard-fought affair, which Muller eventually won 7-5, 6-7, 6-2 in a little over two hours.
Muller meets Agassi in Saturday's other semi-final.