The second seed showed no mercy to world No.13 Monfils - who had his right shoulder heavily strapped - to close out the match 6-2 6-1 in just 60 minutes.
The world No.12 will meet the winner of Saturday's other semi-final between top seed Andy Roddick and fourth-seeded Czech Tomas Berdych.
He also ended a hoodoo of sorts against the Frenchman with today's resounding victory.
Third seed Monfils, 23, held a 3-1 head to head record against the Czech - the only loss came when he retired with injury in 2007.
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Whether it was his tough tournament to date - he had endured three straight three-setters - or his strapped shoulder, Monfils looked out of sorts against the relentless Czech.
The match went with serve until Stepanek broke in the fifth game of the first set - then it was all one-way traffic.
In all he won nine straight games to claim the first set in 33 minutes and race to a 5-0 lead in the second.
Monfils finally held serve - for the first time since the fourth game of the first set - to make it 5-1 in the second, breathing a big sigh of relief amid scattered applause from a packed Pat Rafter Arena.
But there was no stopping Stepanek.
