A bogey-free round helped the Finn to a 3&1 triumph over world number five Stenson, and Ilonen insists that he was always confident he could come out on top at the London Club in Kent.
"The game plan was not to make any mistakes, you can't afford those with Henrik," the 34-year-old revealed to reporters.
"I felt really comfortable going into the final. I felt like I had a good chance of beating him and I did."
Ilonen overcame Dutchman Joost Luiten in the semi-final earlier on Sunday.