Luke Donald was delighted by his start but admits he still has work to do if he is to push home his advantage at the rain-interrupted US Open at Merion.
Donald birdied three consecutive holes from the 11th to move to four-under and the top of the leaderboard through 13 before bad light ended play for the day.
He sits one clear of clubhouse leader Phil Mickelson and Adam Scott, who has played 11, after thunderstorms forced a three-and-a-half hour delay in Pennsylvania, but he acknowledges he must still navigate the course's most difficult stretch to complete his round.
"Those are kind of the holes you want to make an easy score if you can," world No 6 Donald said of the 11th, 12th and 13th.
"Obviously the weather conditions are making the course a lot softer, and you can attack the pins a little bit more. So it's playing as gentle as it might play so far, and obviously this afternoon there was not much wind.
"If you were going to make a score, today was a good day. I've obviously got five holes left and five pretty tough ones to finish, but I'm really happy with the way I started my round."
"I'm excited to get off to a great start," the 35-year-old said. "I made a great putt on one, another good birdie on three, and then just played the holes like they should be played.
"I feel like I'm in pretty good control of my game, and it's nice to get off to such a good start."