The course at the Augusta National Golf Club has been hit by storms over the past 48 hours and further showers are expected tomorrow.
As a result, three-time winner Mickelson has admitted that an alteration in his tactics is likely.
"It seems that some of the planning I have made may go by the wayside," he said. "It's just not the same Augusta.
"It's wet around the greens, and there's no fear of the course. You've got to attack it this week."
Mickelson last won the tournament in 2010.