The Englishman authored a stunning 8-under 62 Saturday at TPC River Highlands to open a four-shot lead over Russell Henley with Day among a group trailing by six.
Day signed for a 4-under 66 to move from T11 to T6 with one round to go and is on track for his best finish in four starts here.
Matt Jones, who began the day in a tie for second and played in the last group, struggled to an even par 70 and dropped to T11.
The real interest is at the top of the leaderboard, however, as should Casey stumble Sunday a logjam of 19 players sit within four shots of second placed Henley.
Among their number are major winners Day, Bubba Watson and Rory McIlroy but a slip up from the former European Ryder Cup star seems unlikely given his extraordinary play to date.
In style!@Paul_Casey shoots 62 after this incredible approach.June 23, 2018
Casey found all 18 greens in regulation Saturday to bring his tally to 23 consecutive after he also found five straight to finish Friday’s second round.
“That’s something I rarely see from me, 18 greens in regulation,” Casey told CBS’ Amanda Balionis after the round.
During that same interview he credited coach and fellow CBS commentator Peter Kostis for his improved ball striking after he found himself struggling early in the week.
“It was a strange one because I texted Peter, not in panic mode, but I had the big pulls going,” he said.
“Every long club in the bag was going 5 or 10 yards left it felt like. We worked on it hard yesterday afternoon after the round and it was pretty simple.
“My posture wasn’t good enough and I was standing too far from the golf ball. So those changes…literally, he had me on the range thinking I was going to shank everything yesterday but whatever he told me clearly worked.”
Casey has been one of the game’s best players for more than a decade and a half though not a prolific winner.
However, his second PGA Tour title came earlier this year when he held off Tiger Woods at the Valspar Championship and his third is well within his grasp with one round remaining in Connecticut.
GENESIS GOLF LINK CUP FEATURE CLUB - COMMONWEALTH GC, VICTORIA:
Want video tips delivered straight to your inbox? Subscribe to iseekgolf.com newsletters.