At the only Champions Tour event held annually in Canada, Labritz surged out to an early advantage, sinking six birdies over his first 17 holes before grabbing one more on the final hole to wrap his bogey-free day.
"I lost my full status two years ago, so I've been grinding back the last few years," Labritz said after Friday's round. "We started out with a nice round today, 63, I feel good, body feels good, I'm ready to go one shot at a time and just see where it ends."
That could have been enough for sole possession of the first-round lead at Canyon Meadows Golf and Country Club, had defending champion Green not capped his five-birdie day with an eagle on 18.
It was a welcome development for the 55-year-old Green, who despite five top-10 finishes this year hasn't managed to duplicate last year's win at this event - his lone PGA Tour Champions title - since.
Labritz, also 55, hasn't competed in the same volume of events this season and has yet to finish higher than a tie for 16th.
Seven more players are within two shots of the lead.
Spain's Miguel Angel Jimenez is a stroke back alone in third at six-under. His day began with four birdies over the front nine before he cooled a touch with three birdies and a bogey on the back.
Six golfers are tied for fourth at five under. That group includes Michael Allen, Jason Caron, Ryan Armour, Jerry Kelly, Michael Block and Thailand's Thongchai Jaidee.
Australians Michael Wright and Mathew Goggin, both with 66s, are tied 10th, one ahead of Mark Hensby.
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