Rookie Andrew Loupe leads the way at the Monterey Peninsula Country Club after he shot a first-round 63 before the heavens opened.

The 23-year-old American - who has not made the cut on his five Tour appearances prior to this AT&T Pebble Beach National Pro-Am - carded an eight-under round to open a two-shot lead.

Loupe was faultless in not dropping a shot and holds an advantage over a chasing pack of five, made up of Stuart Appleby, Jimmy Walker, Jim Renner, Scott Gardiner and Richard H Lee.

Forty players were left with incomplete first rounds after a three-hour rain delay which eventually turned into bad light, with three-time winner Phil Mickelson among them. He is well placed on five under after 15 holes.

Also on that score is the European Ryder Cup captain, Paul McGinley, making a rare appearance on American soil. He completed his round.

England's Brian Davis also rested on five under with two holes left to play, with Dustin Johnson and Hunter Mahan both signing for four-under rounds of 67.

Graeme McDowell was one under and Padraig Harrington level after disappointing displays.

The day belonged to Loupe, though.

"I think it's settling in," he told the PGA's official website of his experience.

"You're not putting next to the guys you've been putting next to. You're not hitting balls to the guys you've been hitting balls next to. I'm looking at guys I've been watching on TV, you know, and it's just realising that I'm supposed to be here.

"I'm here for a reason and I think I was a little nervy the last couple weeks and I thought I did a lot better job of that today."