The Australian will complete 12 seasons of racing in Grands Prix when he steers his Red Bull car around the Interlagos circuit for the final time.
“I’m on a little bit of a slippery slope now,” said Webber, 37, who will go sportscar racing with the Porsche team next season.
“You’ve to be careful not to test it too much in terms of your performance and what you used to be able to do.
“I still think I’m driving well but I don’t want to be around not driving well.”
Popular and respected for his affability, approachability and honesty, Webber said he had not allowed the finale to his F1 career to hit him emotionally.
“I think on Sunday it’ll be a little bit different but it still feels like a normal race at the moment,” Webber said. “I’m looking forward to Sunday in many ways, in terms of pushing for a good result.
“But also I’m ready to stop and looking forward to the extended winter that I’ll have and the new challenges around the corner.
“Getting out of the car on Sunday there will be a few things that will be for the last time in terms of Formula One, but I’m pretty relaxed and looking forward to the race.”
Typically blunt, he added: “I wouldn’t be leaving if there weren’t things I was happy to leave behind. If there were more positives than negatives, then I would stay.
“I still think there are certain situations in Formula One that are super-rewarding. Obviously driving the car on the limit at certain venues is still very satisfying, no question about it.
“Suzuka, Spa, Monte Carlo, come qualifying and even racing at some circuits it’s very challenging and rewarding so I’ll miss some of that.
“It’s inevitable that you’re going to miss certain parts. The adrenaline and working with people like Adrian Newey, you don’t get to do that very often.
“But there comes a time when you’ve got to let go. I’ll still have good adrenaline next year with Porsche and that’ll be a good balance.”
Meanwhile four-time world champion Sebastian Vettel rejected suggestions he and Webber do not enjoy a good relationship.
He stressed how successful they had been as a pairing and how much he would miss the Australian as a team-mate.
“Obviously we didn’t always have the best relationship on a personal level,” he explained. “But in terms of working professionally together for the team, we have been very strong.
“Both of us have tried very hard to improve the car. The fact that he will not be around next year will surely be a loss for the team and a loss for myself.
“I’ve learned a lot from him. I can stand up straight and say that there were many times and places where I benefitted from him.
“You could obviously say the same thing the other way around.”
Relations between the 26-year-old German and Webber have always been competitive, if not strained, notably when they were both involved in the 2011 title battle and also when Vettel disobeyed team orders to snatch victory from Webber at this year’s Malaysian Grand Prix.
“You know, generally people have a worse impression than the relationship that we have,” said Vettel.
“People tend to forget how successful we have been. We’ve been one of the strongest pairings in Formula One.”