Tom Watson, the only golfer to play in all five U.S. Opens at Pebble Beach, said the leaders are going to be challenged on the greens late in the day.
"These greens are like putting over a herd of turtles," Watson said after shooting a 1-under 70.
"The greens are going to be tougher. The backs of those turtles get higher and higher. And the winds will come up and it will dry out the lower parts of these greens. The higher parts will rise up and ... it will get more bumpy."
Added Brandt Snedeker, another early finisher: "The greens are definitely growing with this sunlight. It's going to be fun this afternoon with these guys. It's going to be a hard day for them.
"The greens are starting to grow. They're going to get bumpy. They've got some borderline pins out there, I should say to be politically correct. But they're going to be playing really, really tough this afternoon."