Defending champion Mickelson tightened his left-handed grip on the title with a five-under-par 67 in Saturday’s third round to open up an eight-stroke lead.

Americans Zach Johnson and Jonathan Byrd share second place at 13under-par 203.

"I don’t know where he’s playing," 2004 champion Johnson told reporters of Mickelson’s "freaky good golf" over the last three days.

"He’s obviously driving it very well, hitting his irons very well and putting well. That’s a hard combination to beat, especially when you combine his talent in general and his experience.

"You always want to say there’s a chance but it’s kind of hard to say that in this situation," added Johnson, who shot a best-of-the-day 64 to climb into a share of second.

"At the same time, you’ve got to keep playing. That’s why we play the game. You never know what can happen.

"I’m not going to say he’s going to shoot over par tomorrow but who knows what he’ll shot.

Byrd, who birdied six of the last seven holes, took some encouragement from Mickelson’s faltering finish.

The world number four bogeyed the par-three 16th and dropped two shots at the par-five 18th after twice finding water.

"I guess he made double on the last hole, so that shocks me enough right there that he made that kind of mistake with that lead," Byrd said after carding a 66.

"You never know what’s going to happen out there. To have the kind of run like I had on the back nine today, you just never know.

"But he’s playing some good golf and I’m just going to go out there tomorrow and play the golf course. If I get it going, I get it going. If things start going your way, you just kind of ride it out."