Phil Mickelson believes the USPGA leaders will find life tough going on Saturday, and not just because he is breathing down their necks.
The American shot a second-round 72 to sit four shots behind JB Holmes, who is out on his own at the top of the leaderboard at the halfway point.
Mickelson, the 2005 PGA champion, will find himself in a good position to push for a fourth Major when he tees off with Steve Flesch in round three.
However, the left-hander believes the biggest problem for those starting in the afternoon will not be any player in the field but the firm greens.
Difficult
"I think it gets more difficult in the afternoon, so the leaders are going to have a harder time tomorrow," Mickelson said.
"As the greens get firmer or crustier and bumpier and the ball doesn't check and you can't hit it firm enough, all of the guys who are in the lead and the top half of the field will have a tougher course tomorrow afternoon."
The tough nature of the Oakland Hills course has certainly made it hard work for the competitors.
For the second successive day the scoring average was nearly five over, something Mickelson puts down to several factors, one of which is the inability to control spin on the greens.
Firm and fast
"It's similar, very similar to what we see at Opens normally," the 38-year-old said. "I think that the raking of the grain of the grass away from the green has made it very difficult.
"And given the fact that the greens are firm and fast, and you can't control it, you can't control your spin, it is difficult to get up-and-down.
"I have really struggled around the greens."









