"I think Vandoorne is going to do well, he was really quick in my car in testing," Frijns told AUTOSPORT.
"He is like me [in 2012], a rookie, so he is more free than some of the other guys because he doesn't have to win the championship in the first year.
"The more experienced guys have a lot more pressure to win the championship."
Vandoorne earned €500,000 towards a FR3.5 budget after winning his two-litre crown, and also tested for DAMS during the off-season. He said choosing Fortec was an 'easy' decision however.
"My choice was quite easy between the teams I have tested for in FR3.5," Vandoorne explained.
"I felt very good with Fortec straight away, and we showed some really good speed during the post-season tests.
"I'm happy that finally the deal is done, and I can start focusing on next year. The step from FR2.0 Eurocup to FR3.5 was quite big; the FR3.5 car has a lot more downforce and power compared to the FR2.0 car, but it's a car I like to drive and I feel very comfortable in it."
Team manager Jamie Dye tipped Vandoorne to feature in next year's title fight, adding: "He has the attitude and drive to make his mark on the championship and I'm extremely confident he will be one to keep an eye on next season."
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