(Reuters) - U.S. Open champion Dustin Johnson sleep-walked through his first four holes before snapping to attention to fashion a one-under-par 71 and remain tied for the lead after the second round at the RBC Canadian Open on Friday.

Johnson, who shared the overnight lead with fellow American Luke List after an opening 66, had an early tee time. He started at the 10th hole and bogeyed 11, double-bogeyed 12 and bogeyed the par-five 13th before turning on the jets to salvage his round.

"I just got in some tough spots," said Johnson, who finished the day on seven-under 137 to share the halfway lead with List at Glen Abbey outside of Toronto.

"I think I was still asleep the first four holes but I finally woke up and played pretty well the rest of the day," the world number two added.

List began another hot, dry day at the fast-running, dusty layout with a two quick birdies but ran into trouble on a trio of holes in the valley of the back nine, making three bogeys in four holes.

"I'm real happy the way I finished up," said List, who hauled himself back into a share of the lead with birdies at the par-five 16th and 18th.

The co-leaders held a one-shot lead over Spaniard Jon Rahm, American Kelly Kraft and Canadian amateur Jared du Toit, who all shot 71, the same score the two leaders.

Rahm and du Toit, trying to become the first Canadian to win the championship since 1954, were college team mates last year at Arizona State University.

World number one Jason Day, the defending champion, also had trouble following up his opening 69 as he stumbled to a four-over 76 that left him eight strokes back.

The red-hot Johnson, who followed his U.S. Open win with a victory in the WGC Bridgestone and a tie for ninth at the British Open, said conditions were more difficult on Friday.

"The wind today almost feels like it's a little bit heavier and it's affecting the ball a little bit more, and it's a little bit different direction," said the long-hitting American.

"You've just got to take advantage of the downwind holes and the into-the-wind holes, just try to get looks."

(Reporting by Larry Fine in New York; Editing by Andrew Both)