On a tricky day for scoring at a sun-baked Riviera Country Club, American Haas fired a sizzling seven-under-par 64 to take a firm grip on the tournament.
Aiming to become the eighth player to win back-to-back titles at the US Tour event, Haas produced flawless golf on a firm and fast-running layout to post a 12-under total of 201.
The 30-year-old narrowly missed an eight-foot birdie putt at the par-four last and had to settle for matching the week's best score, Matt Kuchar's 64 in the opening round.
"I'm very pleased obviously," Haas told reporters after soaring to the top of the leaderboard with five birdies and one eagle, a chip-in from 58 feet at the driveable but treacherous 10th.
"I was just kind of plugging along out there and then all of a sudden I made a nice putt at nine, a chip-in at 10 and a good up-and-down at 11.
"I certainly would have liked to make that putt on 18 after a nice drive and a nice second shot but, overall, I'm very pleased."
Fellow American John Merrick recorded two eagles and trailed Haas by just two shots with one hole to play before bogeying the last for a 70 to slip back into a three-way tie for second.
Also at nine under were US Open champion Webb Simpson and in-form South African Charl Schwartzel who both returned 68s.
"Bill shooting seven under is incredible out there," American Simpson said after mixing four birdies with a lone bogey. "It was very tough. I felt like my three under was pretty good.
"It was firm and fast, and the back nine especially reminded me of a US Open, just the way the ball is releasing.
"And the temperature got up, which made the ball go even further, so it was difficult to get the ball close to the pin, especially (to a) front pin."
World number three Luke Donald signed for a 70 to sit four strokes off the pace at eight under with Swede Fredrik Jacobson (72), who was one of two co-leaders overnight.
Fan favourite and twice champion Phil Mickelson, who attracted the biggest galleries, experienced a roller-coaster day on the way to a 72 and a three-under total.
However the bogey-free performance by Haas was the highlight of the day as his seven-under round eclipsed the next-best score of 67, posted by compatriot Stewart Cink.
Ten players returned 68s on a day when the average score was 71.85, almost one over par.
Haas began the day four strokes off the pace but birdies at the fifth, seventh and ninth, where he sank a tricky 10-footer, put him briefly in a four-way tie at the top with Merrick, Schwartzel and South Korean Bae Sang-moon.
Having reached the turn in three-under 32, Haas spectacularly holed out for eagle at the 10th to surge two strokes clear as deafening roars echoed around the course.
"I had a really nice lie and in that grass, you can really spin one," Haas, who won last year's title in a three-way playoff with compatriots Mickelson and Keegan Bradley, said of his chip-in. "And it came off just like I wanted."
Another birdie followed at the par-five 11th, where he chipped up to five feet and knocked in the putt to stretch his lead to three.
Haas parred the next five holes before coming agonisingly close to an eagle at the par-five 17th where he splashed out of a greenside bunker, his ball settling inches from the cup.
He tapped in for birdie to remain three ahead of the chasing pack and maintained control with a rock-solid par at the tricky uphill 18th.
- Sports & Recreation