Sam Burns, a 24-year-old American seeking his first US PGA title, fired a bogey-free five-under par 65 to grab a two-stroke lead after Friday's second round of the Houston Open.

Burns hit 15 of 18 greens in regulation on his way to standing on seven-under 133 after 36 holes at Memorial Park, where 2,000 spectators a day watch under strict Covid-19 safety protocols.

"It takes a lot of discipline around here," Burns said. "You get out of position, you just try to get it back to where you can get it up and down."

Australia's Jason Day, the 2015 PGA Championship winner, and Mexico's Carlos Ortiz shared second on 135.

South African Dawie van der Walt, Canada's Corey Conners and Americans Brandt Snedeker, Patton Kizzire and Aaron Wise were another stroke adrift.

World number 179 Burns birdied three of the first four holes, then added birdies at the par-5 eighth from 11 feet and 12 feet at the par-5 16th on his way to matching the course record.

"Fortunately early we got some good looks and was able to convert," Burns said. "So solid. Looking forward to the weekend."

Burns shared seventh at the Safeway Open in September, where he had his only prior PGA end-of-day lead after round two and learned to be patient.

"Slowing down a little bit, really talking through the shot with Travis (Perkins, his caddie) and making sure we have a clear picture of what we're trying to do," Bruns said. "That was really helpful to learn."

Day, who turns 33 on the opening day of next week's Masters, finds himself playing a patience game as he chases a 13th career PGA title.

"It's very difficult. if you miss the fairway it's hard to reach the green," Day said. "And the greens are very penal.

"If you've got a good short game this week, that helps. You just can't be aggressive around this course. I'll try and stay as patient as I can over the next two days."

Van der Walt, twice a winner on the European Tour in 2013 in his homeland, birdied three of the first four holes and two of the last three to jump into contention as a sponsor exemption starter.

"I'm very happy to play well. It's nice to get to the weekend," he said. "I've been rolling it really nicely. Around this course you've got to make some putts."

No. 1 Johnson fires 66

Top-ranked Dustin Johnson, in his first tournament since contracting Covid-19 and missing two October events, fired a 66 to stand on 138.

"Just a little rusty, but I felt like I swung it pretty well," Johnson said. "Drove it well, hit a lot of quality shots so very pleased with how I played.

"If I can go out and shoot a good score tomorrow, I'll have a good chance come Sunday. I'm just looking to get a little better each day. I'm happy with where I'm at."

Four-time major winner Brooks Koepka, trying to find form after hip and knee injuries, birdied two of the last four holes to shoot 70 and make the weekend on 142, one under the cut line.

South African Charl Schwartzel, clinging on the cut line in his final fairway at sunset, will complete his second round Saturday, the lone player not to finish.

Five-time major winner Phil Mickelson fired an quintuple-bogey eight on the ninth, his highest career score on a par-3 hole. He shot 73 to stand on 149 and missed the cut.