Bryan and Gribble both teed off early at PGA National in Palm Beach Gardens, Florida, and produced six-under par 64s that none of the later starters could match.
But Gribble and Bryan, who is coming off a career-best tie for fourth at Riviera Country Club on Sunday, have no margin for error with Germany's Kaymer and India's Anirban Lahiri just a stroke back, with seven more players on 66.
Kaymer finished with back-to-back birdies for his 65, chipping in from the green-side rough at 17.
Lahiri drained a 25-foot eagle putt at the 18th to join Kaymer at five-under.
Also Read
Bryan and Gribble had set the target early.
Bryan played his first eight holes at even par before a birdie at 18, then poured in five birdies in his last nine holes.
"A place that I'm comfortable at, and a golf course I know, they are few and far between out here," Bryan said. "It's my first run at all these tournaments out here. I love this place, love this golf course and I had a great day."
Gribble, who lifted his first PGA Tour title in October with a storming finish in the Sanderson Farms Championship, hit all 18 greens in regulation to seize his share of the lead.
Disclaimer: No Business Standard Journalist was involved in creation of this content