Brendan Pappas posted an 8-under-par 64. He was joined in third place by Jason Bohn, Mike Springer and Ben Crane.
Hendrix, making his first PGA Tour start of the season, stumbled out of the gate with a double bogey on the first. He bounced right back with a birdie at two.
The Clemson All-American got back into red figures with a chip-in eagle at the third. Hendrix kept rolling with a 9-foot birdie putt on the fourth to move to minus-2. He came back with a birdie at No. 7.
'I made double on the No. 1, but came right back with birdie-eagle-birdie. I wanted to stop the bleeding,' said Hendrix. 'I would never have thought I'd shoot 63 after that double.'
Hendrix jumped to minus-5 with another eagle at the eighth. He then dropped in back-to-back birdies from nine. After a bogey on the 13th, Hendrix drained consecutive birdies from the 15th to get to minus-8.
He closed with a birdie at the par-4 closing hole to post his best professional score.
'Things started to click and I started making some birdies,' Hendrix said. 'I had ideal scoring conditions too. I never had a 63 in a competitive round before this.'
Day started on the back nine and opened with a birdie on the 10th. He dropped a shot at the next, but responded with a birdie on the 12th.
The 39-year-old birdied 15, then capped his opening nine with an eagle at the par-4 18th as he holed a pitching-wedge from 117 yards out. Around the turn, Day climbed to 6 under with birdies on one and three.
Day dropped a shot at the fourth when he found the water off the tee, but closed with four birdies over his final five holes to share the lead.
'I played pretty good today,' Day said. 'There are going to be a ton of birdies made between now and Sunday. I just need to keep making birdies myself.'
David Edwards, Harrison Frazar and Hank Kuehne, who tied for second last week at the John Deere Classic, share 12th place at 6-under-par 66.
Defending champion Jonathan Byrd is in a large group tied for 54th at 2-under-par 70.