Nissan Open:Padraig Harrington rued wasted opportunities after ending the third round of the Nissan Open in California trailing leader Phil Mickelson by one stroke.

Mickelson shot a two-under-par 69 to lead and going into the final round heads the field on 13-under par, with Irishman Harrington nicely poised in second place a shot back.

Rich Beem's 65, the best round of the day, means the American lies third on 11 under par behind as he chases his first title since the 2002 PGA Championship, while Robert Allenby and Charles Howell III trail by three strokes after rounds of 68 and 69 respectively.

Harrington and Mickelson began the day tied for the lead on 11 under par but two quick birdies by the American gave him the advantage.

Mickelson nearly reached the green on the par-five first in two shots, his approach settling on the fringe 97 feet from the hole before two-putting for a birdie.

He added another on the second when he made a five-foot putt, before three birdies and as many bogies on the back nine ensured he remained two-under for the day.

Harrington, by contrast, birdied the third but dropped a shot on the par-three sixth before a birdie on the 11th hole and a run of pars secured a round of 70.

Harrington rued the squandered chance to put distance between himself and his rivals.

"I had a few opportunities," he said.

"It would have been nice to get to 15, 16-under par, so that would have taken the field out."

Mickelson was left to reflect on a missed opportunity to put himself in a strong position.

Sitting on 15 under par after 11 holes, bogies at 12, 13 and 16  gave renewed hope to the chasing pack.

"It could have been a chance for Padraig (Harrington) and I to pull away a little bit there in the end," Mickelson said.

"The three bogeys on the backside let 12 to 15 guys back in the tournament.

This tournament does not traditionally have runaway winners. The last person to win by two strokes was Ernie Els in 1999, while Nick Faldo won the tournament in 1997 by three strokes over Craig Stadler.

Since then there have been four play-offs and with fast greens and pristine conditions, Mickelson expects the same type of ending.

"There were a lot of low scores," he added.

"If Padraig and I had both shot low scores, we probably could have pulled away a little bit. Instead it will be a shootout.

"We have to play a good solid round, it's not like we have to do anything spectacular."

Looking ahead, Mickelson said: "We have to play a good round and shoot under par and probably four or five under I would guess.

"But there are plenty of birdie holes out there to do that."

Beem's 65 included a hole-in-one at the 177-yard 14th and he promptly jumped on the roof of the new bright red Nissan his ace earned him.

"I stole that move from (Peter) Jacobsen,"  he said.

"I wish I could take full credit for making a fool out of myself, but Jake won a convertible and he hopped in it. I will never forget that.

"I got a couple of really good breaks out there to start off the round.

"I hit the ball solid all day and I made a few putts and getting a one always helps."