Buemi eyes Le Mans and Webber’s 2013 race seat
He told Le Matin newspaper on Friday it was the "logical continuation" of his three-year F1 race career.
"Obviously I was surprised and was not expecting the (Toro Rosso) decision at all. But the Red Bull people immediately offered me the job as third driver and I thought about it before agreeing.
"Being close to them will be the best way to get a spot on their team one day in the future," said Buemi.
He admitted, however, that F1's limited track testing means he may have to "find a complementary Program" elsewhere.
"Ideally I would couple F1 with a different one, such as endurance — I like the Le Mans 24 a lot.
"There is no grand prix on that (Le Mans) weekend, so it would be possible. The big teams will start their testing soon, so I could participate as well," said Buemi.
"I will probably be able to announce the details of what I will be doing in 2012 fairly soon." Beyond this year, he insists his future is completely open.
"The contract I've signed covers just one year. After that I have not been promised anything so we'll see.
"Actually I think it's better to be the third driver in the best team than to race at the bottom of the grid. Since December I was in contact with other teams as I had opportunities elsewhere. "But ultimately the best solution was to stay with Red Bull. Now it's up to me to do my best in the simulator to prove to them that I am qualified in case Mark Webber's contract is not renewed for 2013," added Buemi.