The FedEx Cup finale is set for a thrilling finish at East Lake Golf Course after Cantlay's six under 64 saw him join PGA Tour nearly-man Fleetwood at the top of the TOUR Championship leaderboard, two ahead of Russell Henley.
"It's nice to close the way I did, especially the last two days," Cantlay said afterwards.
"I'm pleased with where I'm at. It's always nice to have your game shape up at the end of the year, but like I said, I'm focused 100% on this week, and we'll worry about the Ryder Cup a little later."
Fleetwood signed for a colourful 67, which included seven birdies, a bogey and double boy fave when his ball found the water on the par three 15th. The Englishman missed a birdie chance on the last to separate himself from Cantlay.
Meanwhile, US Ryder Cup captain Keegan Bradley lurks alone in fourth on 13 under, three of the pace. No matter what happens on Sunday, Bradley may punch his own Ryder Cup ticket as a playing captain for the Americans.
Further down the leaderboard, FedEx Cup leader Scottie Scheffler is alone in fifth on 12 under.
"Felt like another frustrating day," said the world number one.
"I felt like I did some things well out there. I was just a little bit off on my reads most of the day. I hit some good putts that just didn't fall. Overall, I didn't shoot myself out of the tournament, which making bogeys around this place can very easily do that. Proud of the turnaround," he said. "But man, I wish I could have gotten a little bit more out of the round."