The drivers were teammates at McLaren during the Briton's first season in the sport in 2007.
The 28-year-old Mercedes driver told the Daily Mail: "Talking about a rivalry with Jenson [Button] doesn't really excite me, the rivalry with Fernando excites me more.
"He's the guy I want to beat. You also want to beat [Sebastian Vettel], as they are the ones with most titles. But Alonso's the fastest driver I can see. He's also one of the most experienced.
"Anyone would struggle to beat him. You've got to try and stay ahead of him in a Ferrari that's competitive. That's going to be a real challenge."
The 2013 F1 season gets underway next weekend.