The 17-time Grand Slam champion has suffered a decline this season, underlined by early exits at Wimbledon and the US Open, while also slipping to seventh in the rankings.
In contrast, the Spaniard has enjoyed a remarkable comeback season, winning at Roland Garros and Flushing Meadows on his way to retaking the number-one ranking from Novak Djokovic.
"I just feel like I need to look at it more as being an underdog a little bit because of circumstances, because of my year, because of his year," Federer told Sky Sports News.
"Maybe that free swinging is what I need to do a little bit more. We know each other that well.
"Hopefully it's going to match up good for me. If not, he deserves it. He's had a wonderful season."
Nadal holds a 21-10 head-to-head advantage over the Swiss star and has won their last three meetings.