Putnam is playing his fourth PGA Tour event. He earned his tour card this year through the Web.com Tour.
"It was a good round,'' said Putnam, who birdied all three par 5s. "I started off slow, one bogey, but played good out there. I played the par 5s well today, was able to take advantage of those.''
Knox, from Scotland, had a 67 to match Putnam at 10-under 132 at TPC Summerlin.
Knox had six birdies and two bogeys -- on Nos. 2 and 3 -- in the second round.
"I got a bit worried, though, because I was all over the map,'' Knox said. "I didn't feel real good with my swing, and I knew if I just made some pars, eventually something would click. And I made a couple putts, which really settled me down.''
"I putted really well today,'' Svoboda said. "I made some long putts.''
Defending champion Webb Simpson topped the group at 8 under after a 65.
"I just kept it right in front of me,'' Simpson said after his bogey-free round. "That's what I did last year on this golf course. If you keep hitting the fairways, you're going to have opportunities to make birdie. I made some putts today. Didn't make many yesterday. All around, just a good, solid day.''
Spencer Levin, George McNeill, Ben Martin, Bo Van Pelt, Scott Piercy and Wes Roach also were 8 under. Levin and McNeill shot 63 -- the best rounds of the week. Levin birdied seven his last eight holes.
"I was 1 over through three, so to shoot 8 under is pretty cool,'' Levin said. "I made a couple nice putts on my 17th and 18th holes, which kind of makes you feel a little better. I really didn't play that bad, I just didn't putt well yesterday. I probably played a little better today tee to green, but not much. It's just the putts went in today.''
McNeill birdied seven of his last 10 holes.
"Obviously, I knew what I had to do,'' McNeill said. "I wasn't trying to really force anything, but fortunately got off to a decent start, and again in the middle of the round kept it going, and then a nice finish birdieing 18.''
Cink followed his opening 64 with a 75 to drop into a tie for 45th at 3 under. He had a quadruple-bogey on the par-4 11th -- his second hole of day -- and made a bogey on the 12th. He also had a bogey on No. 6 and a double-bogey on No. 7.
Stuart Appleby went the other direction, rebounding from a 74 with a 64 to move into a tie for 30th at 4 under.
Jarrod Lyle had a 71 to fall into a tie for 22nd at 5 under. The Australian is making his second PGA Tour start since a recurrence of leukemia some 18 months ago.
FedEx Cup winner Billy Horschel missed the cut with rounds of 71 and 72. At No. 13, he was the top-ranked player in the field.
Sang-Moon Bae, the Frys.com Open winner Sunday in the season opener in Napa, California, also shot 71-72 to miss the cut.