A two-time victor on the Web.com Tour, Putnam birdied No. 18 to tie up Johnson once again after a bogey-free round.
Dustin Johnson wants to sharpen his game for the U.S. Open. Putnam's solitary birdie of the round came two holes later to narrow the gap to two shots, but he wouldn't come any closer than that for the remainder of the round.
Dustin Johnson already had his 18th career PGA Tour victory in his grasp, but he made a decision to end his day with an exclamation point anyway.
Cook's 5 under on Sunday tied him with Phil Mickelson and Tim Herron for the best scores of the final round and moved him up the leaderboard into a tie for sixth.
Johnson, who won the U.S. Open in 2016, heads to Shinnecock Hills after stringing together four straight rounds in the 60s.
NOTES: Johnson was the first on tour to win by holing out from off the green since Jordan Spieth at the 2017 Travelers Championship. Snedeker turned in a bogey-free round with eight birdies needing only 22 putts. "It was a long way to get there, and I held it for a long time and obviously JT took it from me for a little while". Now he will play in the last group on the final day for the first time on the PGA Tour with Johnson.
Dustin Johnson, tied for the third round lead with fellow American Andrew Putnam at the FedEx St. Jude Classic in Tennessee, will regain the world number one ranking if he wins on Sunday. I know if I can drive it in the fairway that I'm going to give myself a good look for birdie. "I hit a lot of good shots coming in". He holed out from the right front bunker for birdie on the par-5 No. 16.
Johnson put an exclamation point on his victory with a hole-out eagle from the fairway from 169 yards away on the closing hole, his second such hole out of the week.
Putnam, 29, is a two-time Web.com Tour victor in his second full season on the PGA Tour.
It appears on paper as a one-sided battle but tournament golf works in unusual ways and Putnam is an emerging star with two victories on the Web.Com Tour prior to graduation to the bigger stage this year.
Mickelson headed a group of six golfers finishing in 12th at 6 under.
Luke List plays in the second round of the St. Jude Classic golf tournament at TPC Southwind, Friday, June 8, 2018, in Memphis, Tenn.