Monfils will face Spain's Albert Montanes, a 6-4, 4-6, 6-0 winner against French wild-card entry Edouard Roger-Vasselin in the first semifinal in the clay-court, warm-up tournament for the French Open.
A former top-10 player, Monfils was hampered last year by a right knee injury and has dropped to 109th in the rankings.
Monfils hit 17 winners to eight for Andujar to take the first set, when the Spaniard netted a backhand volley. The Frenchman then rallied from a 4-2 deficit in the second.
The French Open starts Sunday.