The 24-year-old told Autosport.com: "Before all the Red Bull talk, I had come off two poor races. That alone, and the fact that [Jean-Eric Vergne] had two really strong races, which showed the car had potential, gave me a lot of fire.
"I like to think I always work hard away from the cockpit, but maybe I worked even harder. That was the real motivation to make me step up, the Red Bull thing was just a bit of icing!"
Ricciardo followed up an eighth-placed finish at Silverstone with a 12th at the German Grand Prix, while Vergne suffered back-to-back retirements.