The last time Woods won by more than six strokes the week before a major was in 2007 at Firestone, just ahead of his PGA Championship title at Southern Hills.
"I had a totally different golf swing back then compared to now," Woods said. "Performance-wise, yeah. Scoring-wise, yeah. But for me it's hard to relate because it's a totally different emotion."
Mickelson, a US Open runner-up for the sixth time in June, captured his fifth major crown in July and is confident he knows Oak Hill well.
"I've studied the golf course," Mickelson said. "I know exactly how I'm going to play it. I just need to get my game sharp now," Mickelson said.
Nicklaus, who won the 1980 PGA at Oak Hill, sees Woods and Mickelson as favourites but warns a lot of players could be in the hunt because the course will force golfers to change their games.
"Basically I'll just try and get a feel for the golf course and how it's playing," Woods said.
"If you have an opportunity to make a birdie, you had better because there aren't a whole lot of opportunities to make them.
"The rough was already up when I played it on Tuesday [last week]. It has another week of getting thicker and more lush. I think it will be a very, very difficult championship."
Mickelson warned that players need to be short of the greens and below the hole for the best chance at making putts and also marvelled at the dense rough and tight fairways.
"You've got to hit fairways," Mickelson said. "The rough is extremely long and thick."
- AAP