The Mexican player holed a 100-yard wedge shot for eagle at his first hole and never looked back as he blitzed the TPC Deere Run course in Silvis with a nine-under-par 62, to lead Americans Adam Long and Russell Henley by two.
After the top three on the course, there is a sizable tie at 6 under which includes Martin Laird, Ryan Palmer and Vaughn Taylor.
"I just feel that everything clicked", Diaz said.
But 200th-ranked Henley, whose best finish this year has been a share of 15th at Phoenix, found the rough on his first two shots at nine, which played as the second-toughest hole of the day, and closed with a bogey to fall level with Long. I've been hitting a lot of fairways, and today I hit good numbers all day, and that helps. Henley, a back-nine starter, made eight birdies, most after putting his approach inside four feet and the last a 48-footer at the par-3 seventh.
The 19-year-old Chun, also seeking her first LPGA Tour victory, had a bogey-free round.
Now 167th on the FedEx Cup standings Davis has a lot of work to do in the next six weeks or so if he is to safely retain the status he gained via the Web.Com Tour finals in 2018 but this was encouraging for the 23-year-old.
Twenty-year-old rookie Matthew Wolff, who picked up his first career win at the 3M Open last week, opened with a 67.
"Having that PGA Tour card locked up is a lot of weight off of my shoulders", Wolff said.
"I've been playing pretty good the whole, since February I've been playing really solid".
Meanwhile, defending champion Michael Kim posted a first-round 73 to be in a tie for 134th.
Jonathan Byrd, the 2007 victor, is tied for 78th at 70 and 2012 champ Zach Johnson champ is tied for 123rd at 71. Johnson, who last shot over par there in 2008, missed a long birdie putt that would've kept the streak alive by inches.
"It's still golf, it still drives you insane, and I still have a bit of a perfectionist in me that I don't like to settle for mediocre", he said. He's playing the 50-and-over event instead of going for a fourth victory at the John Deere Classic on the PGA Tour.
Paul Goydos was second at 2 under after five holes.