Keith Mitchell held the first-round lead at three tournaments earlier this season before falling out of top spot. He's showing staying power at the PGA Tour's Truist Championship.

Mitchell followed a career-best round with a three-under 67 on Friday and maintained a one-shot lead after two rounds at Philadelphia Cricket Club's Wissahickon Course.

The American, who led the way with a 61 on Thursday, offset two bogeys with five birdies for a 12-under total.

Shane Lowry (65) is second at 11 under and Austria's Sepp Straka (67) another stroke back. Defending champion Rory McIlroy (67) sits in a large group for fourth.

"Playing well the last couple Thursdays, it's just I feel like I've been there now three or four times. So trying to just build off of that and build into Friday, then Saturday and Sunday," Mitchell said.

"It's a four-round tournament. They don't give any points or money out on Thursday. So I've got to keep it going."

One day after 64 of the 72 golfers in the no-cut field broke par in near-perfect weather and scoring conditions, the course was softened by overnight rain and the field dealt with on-and-off showers.

Lowry was undeterred by the weather and posted the lowest round of the day.

He birdied four of his first eight holes. After a bogey at the 10th, he bounced back by making a 53-foot birdie putt at No.11 and closed out the round with another birdie 15.

The 38-year-old discounted the notion that he has an advantage over most players in chilly, damp conditions just because he hails from Ireland.

"Everyone says that to me every day when it rains," he said.

"I live in South Florida, and I plan to be there now ... I think I'm able to handle them probably better than a few people, but I don't particularly like or enjoy going out and playing in these conditions."

McIlroy had an uneven round of six birdies and three bogeys while playing in his first individual tournament since completing the career Grand Slam at the Masters last month.

He has won three times this season — at Pebble Beach, the Players Championship and Augusta National — and is a four-time champion of this event.

"I felt like today was another sort of scrappy one," McIlroy, who is also preparing for next week's PGA Championship at Quail Hollow, where he has won four times, said.

"I made what I feel are some uncharacteristic mistakes compared to how I've played the majority of the year."

Min Woo Lee (68) is the best-placed Australian at three under, while Cam Davis (69) and Adam Scott (70) are one stroke further back.

With AAP.