Australia's former world No.1 repeated his first-round 66 with another four-under-par score on Friday (Saturday AEST) to jump into outright fourth, just one stroke behind the halfway leaders.
World No.1 Scottie Scheffler (62-69), fellow American and dual major winner Justin Thomas (67-64) and Englishman Tommy Fleetwood (66-65) share the lead at nine under through 36 holes.
Scheffler is chasing a fourth win from six starts, Thomas his second of the season while Fleetwood enhanced his hopes of maiden PGA Tour victory with two eagles and a birdie in a spectacular four-hole back-nine burst.
Day defied extreme winds to conjure seven birdies at TPC River Highlands, including four on a flawless back nine, to be right in the mix for the $US3.6 million winner's cheque.
"There was 40-mile gusts out there, so, you had to hit on the right wind," he said.
"To be able to walk out at eight under - four under, four under - after two days, I'm very pleased with it."
Day continues to enjoy an impressive season, despite freak neck and wrist injuries derailing the former PGA Championship winner's campaign over the past three months.
After chalking up top 10s at The Masters, Arnold Palmer Invitational and The American Express, the 37-year-old had to sit out The Players Championship through illness then The Truist Championship after hurting his neck.
Compounding his troubles, Day then suffered triangular fibrocartilage complex damage to a wrist while moving a bike.
He couldn't play for almost a month between missing the cut for the first time at the PGA Championship and finishing a creditable joint-23rd at last week's gruelling US Open at Oakmont Country Club.
Little wonder he is chuffed to be back on the leaderboard at the $US20 million event in Connecticut.
"I feel pretty good," Day told the Golf Channel.
"Obviously I had the disc protrusion in my neck, and that feels like it's been a year-long thing.
"Unfortunately I haven't been able to hit it as hard as I'd like to. It's just like a little freak thing that happened, and unfortunately the wrist goes with that as well.
"So I had a little TFCC strain and I feel good now, which is great. I'm still trying to manage it all. From here (the neck) down, I feel tremendous.
"Everyone probably knows that I've battled some lower-back issues in the past, and I feel like I've had a pretty good handle on it. Then these two things have just come out of the blue and unfortunately just set me back.
"I missed five out of six weeks, missed two signature events on top of that. Then I really didn't have a great prep going into the PGA, a limited prep going into the US Open, so it's nice to be in the mix coming into this weekend."
Day looks the only Australian chance, with Min Woo Lee (73-69) back in a tie for 49th at two over.
Cam Davis (71-73) and Adam Scott, on a come down from his US Open surge, are four over and equal 59th in the 72-player, no-cut field.
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