"The grass is quite thick," the 43-year-old Clarke said as the rain lashed down at the Lytham and St Annes venue.
"It's not quite what we would expect on a links course. It's a little bit thicker than what you normally find.
"It's really tough - if you start spraying the ball around this week you might as well go home. There are a few patches out there where it's just absolutely brutal."
American Woods was similarly taken aback when he played a practice round at the Lancashire links course on Sunday.
"I've never seen the rough this high or thick and dense," said the three-times Open champion.
"You can't get out of it. That bottom six inches, in some places, is almost unplayable."
Clarke, who has struggled badly for form since winning his first major at Sandwich last year, believes there will be plenty of lost balls this week.
"There are a few places where...I don't know if you'll be able to take a full swing and move it," he explained.
Clarke also said the bunkers dotted around the 7,086-yard, par-70 layout would be almost as difficult as the rough.
"It's a nightmare because there are 205 bunkers and... they're very penal, very tough," said the former Ryder Cup stalwart.
"You know there are going to be occasions here this week where I think you're going to see guys taking penalty drops out of them because they won't be able to move the ball anywhere.
"They are a huge part of this course. It's very heavily bunkered in the landing areas where we really need to hit the ball," added Clarke.
"Accuracy is going to be the key this week. It's not really length off the tee, it's going to be keeping it on the fairways and keeping it out of those bunkers."
Clarke has missed a host of halfway cuts since landing the coveted Claret Jug a year ago.
"I don't know if that's because of winning the Open or not but I've certainly fallen into a little bit of a trap of trying to play better and trying too hard," he said.
"I've been getting into a mentality that I've got to go out and play like the Open champion instead of just playing the way I played in the first place."
Clarke said the trophy was a bit battered and bruised when he handed it back to Royal & Ancient chief executive Peter Dawson on Monday.
"I didn't really want to give it back," said the Northern Irishman. "The year has gone amazingly fast and it's been an honour and a privelege to represent the R&A and take the Claret Jug all over the world.
"It's not quite in as good a condition as I received it in. It's been here, there and everywhere but Mr Dawson looked at it and said, 'Oh, we can fix this, we can fix that', so it's not too bad."