A day after revealing he had undergone stem cell treatment on a troublesome left knee, Koepka was far from his best at TPC Summerlin, trailing early Canadian leader Nick Taylor by seven strokes.
The 29-year-old world No. 1, whose rust was evident, ran up five birdies and four bogeys.
Koepka said stem cells were injected into his knee the day after the Tour Championship in late August.
“Finally feel 100%, which is nice, and hopefully stay that way for the rest of the season,” he said. “I’d been battling it there for the last four months of that season.
“My patella tendon was partially torn ... I also had a wrist injury.”
Those concerns did not stop Koepka from a magnificent season in the majors. He won the PGA Championship, was runner-up at the Masters and U.S. Open, and fourth at the British Open.
Koepka, an especially dangerous competitor when he has a chip on his shoulder, was controversially beaten for PGA Tour Player of the Year in a vote by his peers.
Never mind his major record, Koepka was bested by season-long FedEx Cup winner Rory McIlroy. The tour did not reveal the vote totals.
On Thursday, he was also bested by younger brother Chase Koepka who shot a bogey-free 66 in just his fourth PGA tour start.
Chase said he would relish the chance to let his brother Brooks know he got the better of him.
“I do have to needle him every now and then when I do get a chance to nip him on a day like today. I’m sure he’ll hear it over dinner,” Chase said.
“It’s nice. But, you know, he’s so good. He’s so good.”
Taylor leads by a stroke over Brian Harman after a round of six birdies, an eagle and no bogeys.
Phil Mickelson headlines a group of five players at six-under-par. He is trying to rejuvenate his play with a new healthy lifestyle that his seen him drop pounds in recent months.
“I’m going to continue to make it a lifestyle change,” Mickelson said.
“I’m going to continue to eat better, eat less, work out more, just stay committed to it.” Patrick Cantlay, Bryson DeChambeau and Australian Adam Scott are all at five-under after 66s.