The Scot had already beaten Belgium's Joachim Gerard, ranked four places above him in the world, in the first round 3-6 6-4 6-3.
The 27-year-old wheelchair star broke a five-match losing streak at the slams in the quarter-finals and carried his form into the last four, where he defeated top seed Shingo Kunieda from Japan 2-6 7-6 (2) 6-2.
In the final Reid will face second seed Gustavo Fernandez, who ended hopes of an all-British final by beating Alfie Hewett 6-1 6-2.
Speaking ahead of the French Open, Reid - who helped Great Britain to World Team Cup victory last month - told the BBC: "The last 12 months haven't been great in terms of my form, but I feel like it just takes one event to switch that and this could be the one for me.
"Everybody's level has raised, it's tougher and tougher, and consistency is more important than it ever has been.
"Physically I feel in good shape, but mentally, psychologically, I need to work on a few things to get myself back to the highest level."
The final will be held on June 8 and starts at 9am UK time.