Francis, of Scottsdale, AZ, had five birdies and one bogey. Gribble, of Dallas, TX, playing in his first USGA championship, bogeyed his first hole, but ended with five birdies on the front nine before parring his final 10 holes.
'It's a huge tournament and I'm glad to have such a good start,' Gribble said. 'But, (Tuesday) is a brand new day. It feels good to go in with a 4-under and, so far, have some slack to rest on and play like it's a nice day in California.'
John Douglas Archibald and Arnond Vongvanij are each a shot back, while Rickie Fowler, Wesley Graham, Sean Maekawa, Blake Redmond and Chris Hamm are all at minus-2.
Defending champion Kevin Tway, the son of PGA Tour player Bob Tway, is tied for 33rd place at 1-over 73.
Stroke play finishes Tuesday. The low 64 scorers will qualify for match play, which begins Wednesday and concludes with Saturday's 36-hole championship final.
Winners of this prestigious event reads like a who's who of superstars, starting with none other than Tiger Woods. Ranked as the number one player in the world, Woods captured three consecutive Junior Amateur Championships from 1991-93. Other prominent past winners of this event are: Gay Brewer, Johnny Miller, Gary Koch, Jack Renner, Willie Wood, Brett Quigley and David Duval.