All eyes will be on 3M Open victor Matthew Wolff when the John Deere Classic gets underway at TPC Deere Run.
But thanks to a bogey-free round and an eagle to start his back nine, Diaz holds that two-shot lead over Adam Long and Russell Henley.
There is plenty of incentive for many in the field, including Davis and Henden, with the victor earning a berth at next week's British Open.
Neither Diaz, Henley nor Long are now in the Open.
His best finish in a tour event was a share of eighth three weeks ago at the Travelers Championship.
"Everything clicked today", he told reporters of his round that included seven birdies.
Going out from the 10th tee in Silvis, Il., the 20-year-old Wolff played the first nine holes even-par before he would birdie his next three holes. The driver has been awesome.
Starting on the back nine in the afternoon wave, Henley reached 8 under before his drive on the final hole found the right rough.
Wolff, who left Oklahoma State to turn pro after the collegiate season and the U.S. Open, is playing in just his fifth event as a pro and officially accepted his two-year PGA Tour exemption this week. I've been hitting a lot of fairways, and today I hit good numbers all day, and that helps.
"I don't think I've ever lost the belief I can have a nice tournament and a nice round". "It's just a matter of a few bumps here and there".
Long was a surprise victor at the Desert Classic in California in January, but has done precious little since.
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.
"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. "We have four more tournaments to go, and I'm trying to get into the (FedEx Cup) Playoffs". "I haven't qualified yet, so I'd love to be there".