Mickelson in title hunt at Riviera

PACIFIC PALISADES, California (Reuters) - Masters champion Phil Mickelson, maintaining his impressive form, charged into contention at the Nissan Open on Thursday.

Four days after winning his 30th PGA Tour title with a five-shot victory at the Pebble Beach National Pro-Am, the American left-hander fired a five-under-par 66 in the opening round.

Mickelson, who rattled up five birdies in benign conditions at sun-drenched Riviera Country Club, surged into a tie for second place, three strokes behind pacesetting Irishman Padraig Harrington.

"I'm very happy with that first round," the 36-year-old Californian told reporters after producing one of only two bogey-free scores in the 144-strong field.

"I thought I played very well. I've been playing well coming in and to get off to a good start always feels good."

Asked if his opening round was a carry-over from last week at Pebble Beach, Mickelson replied: "I'm not sure.

"Certainly the greens here are similar but all the shots I was hitting last week, the little knock-down shots in the wind, I couldn't hit today because the greens were so firm.

"But I think the momentum may have helped carry me through, especially on the greens. I putted very well today, and made a lot of five to six-footers."

Mickelson, who needed only 26 putts, applauded Harrington's superb opening 63.

"I hit some good shots today but it is a little humbling to shoot what I thought was a good round and then get lapped by three," the three-times major winner said.

"That was an incredible round that he played. The greens were firm and a lot of the pins were tough to get at."

Mickelson, playing his fifth consecutive event on the 2007 PGA Tour, has not competed at Riviera since 2001 when he missed the halfway cut.