Updated

By John Reger

PEBBLE BEACH, California (Reuters) - Defending champion Dustin Johnson and fellow American Paul Goydos are set for a final round showdown at the Pebble Beach National Pro-Am after shooting a pair of 64s on Saturday to charge four strokes clear.

Johnson, who won last year's tournament by default after the fourth round was washed out by heavy rains, tore up the front nine of the Spyglass Hill course in his eight-under par third round, but was matched by Goydos, who shot a sparkling bogey-free round on the Pebble Beach Golf Links.

The Americans, part of a six-way tie for the lead overnight, marched to an 18-under total of 196 to emerge four clear of fellow overnight leaders, Americans Bryce Molder and J.B. Holmes, and Matt Jones of Australia.

Johnson, who has led or held a share of the lead over the course of the tournament, drained a 20-foot putt to eagle the first then notched four birdies in five holes from the third to reach the turn at 30, matching Phil Mickelson's mark in 2005.

"You know, that's key for me, getting off to a good start," Johnson told reporters. "Eagled the first hole and hit two great shots on holes just right past the flag."

"The length is not an issue, (the course) doesn't play very long for me," he added of Spyglass, considered the toughest of the three played at the pro-am.

"If I'm hitting the fairway then it definitely plays right into my hands because I can get to all the par fives there."

The big-hitting American, who also charged out of the blocks with a front nine 30 at Pebble Beach in the first round, stumbled with a bogey on the 10th, but recovered with an eagle on the next hole and a chip-in for birdie on the par-five 14th.

His hopes of equaling the 62 course record shared by Britain's Luke Donald and Mickelson were dashed, however, with a bogey on the last.

In contrast to Johnson's fireworks, Goydos was all control, turning in a bogey-free round highlighted by an eagle on the par-five second where he holed an eight-foot putt after reaching the green in two.

Driving on average nearly 50 yards shorter than Johnson, Goydos notched six birdies in addition to his eagle to give himself every chance of notching his third tour win in a 17-year career.

"I'm going to appreciate (Johnson's) play, but you go out and play your game too," the 45-year-old said. "I have to do things differently than he does and he's got to do things differently than I do."

Twenty-five year-old Johnson, however, won the 2008 Turning Stone Championship in only his second year after turning professional then clinched his second at Pebble Beach last year.

"He's won twice in two years, I've won twice in 17, better is an interesting word," Goydos said when asked to compare their careers. "He's definitely off to a much better start than I had."

"Last year was last year," Johnson added. "I've got to focus on tomorrow and getting the job done. I'm not going to be thinking about last year at all."

(Editing by Ian Ransom)