Both men opened with 70 on Thursday.
Second-round co-leader Hideki Matsuyama and Chris Stroud, who began his third round tied for second after finishing his second round Saturday morning, are one behind Kisner at 6-under 207.
Day added that Matsuyama's effort has turned putting from a vulnerability into a weapon.
But he recovered quickly, draining a 14m birdie bomb at the par-3 13th before backing up with another birdie on the 14th.
The 29-year-old Australian hit his second shot into a shrub and had to take a penalty drop en route to a crippling quadruple-bogey eight. Stroud shot 71 and Matsuyama checked in with 73.
This will be his first time with a piece of the lead heading into the weekend of a major, and he seems excited for the experience and the challenge. "It's just unfortunate because I was actually in a rhythm and the momentum was going my way and we got called off". "But whether it's the best that I've ever played in my career, I'm not sure", he said.
Speaking of his misses, Lahiri explained, "On the second nine (the front side of the course) I missed two makeable putts from short distances like from six feet on fifth (for par) and from five feet on eighth (for birdie)".
Soren Kjeldsen: Six shots back to open the second round, Kjeldsen began to tumble nearly immediately. "Hopefully he slows down a little bit and gives the rest of us a chance, but I still feel I'm there".
"I'm disappointed the way I played (Saturday)", Matsuyama said via an interpreter. "I have to hit my tee ball where I'm trying to look, and if I don't, I have to find a way to get the ball in the hole the fastest".
Kisner, meanwhile, used his "Texas wedge" from off the green to sink a 47-foot putt at the par-five seventh - his 16th hole - and vault clear of the field, at least for a while.
He finished the day with three birdies, four bogeys and two double bogeys but what hurt even more was the small tap- ins that did not fall and the chips that were way below his standards.
Matsuyama sees his birdie streak snapped with a par at the par-four 16th before resuming his torrid back nine with another birdie at 17 on his way to a flawless seven-under 64 that ties him with Kisner at eight-under 134 through 36 holes. He closed with back-to-back pars.