MEMPHIS, June 7, 2013 (AFP) - Green, who won his only US PGA title at the 2009 Canadian Open, has struggled with his game in recent years.
So finding himself in the lead midway through the round was an unfamiliar experience.
"I was starting to get nervous seeing my name on the board," admitted Green, who tapped in for his fourth birdie of the day at the par-five 16th, then parred the last two in a four-under effort on the par-70 TPC at Southwind layout.
He was joined at four-under by compatriot Stuart Appleby, US veteran Davis Love, Glen Day, Martin Flores and Harris English.
Another 11 players, including defending champion Dustin Johnson and two-time St. Jude winner David Toms, were a stroke back on three-under 67, with another nine on 68.
As crowded as the leaderboard was, four-time major champion Phil Mickelson didn't figure on it.
Mickelson, playing in the $5.7 million tournament with one eye on next week's US Open at Merion Golf Club, carded a one-over 71.
"My short game was not good today," Mickelson said.
Mickelson opened his round with a promising birdie at the par-four 10th. But he bogeyed 15, where he missed the green and pitched 30 feet past the hole.
His second shot into the par-five 16th found a greenside bunker and he hit over the green with his third shot. After landing in the sand again, he ended up with a double-bogey seven, and followed that with a bogey at 17.
He clawed back with three straight birdies starting at the first, but capped his round with one last bogey at the ninth.
"I really feel like I'm striking the ball well," Mickelson said. "The score obviously isn't very good... I'm looking forward to (Friday's) round because I know that ball striking-wise, it's about as good as I've been striking it and I feel like there's a good round in me."
Green was the only one of the leading group to play without a bogey. He admitted it was a welcome change of pace.
"Last few years, my game has gone a little bit south," he said. "I was always a really good chipper and putter, and I lost that part of my game almost totally. Am sort of rebuilding that."
Leading first-round scores on Thursday in the $5.7 million US PGA Tour St. Jude Classic (USA unless noted, par-70):
66 - Nathan Green (AUS), Martin Flores, Davis Love, Stuart Appleby (AUS), Glen Day, Harris English
67 - Nicholas Thompson, Scott Stallings, David Toms, Jason Bohn, Dustin Johnson, Peter Hanson (SWE), Tim Petrovic, John Rollins, Justin Hicks, Paul Haley, Shawn Stefani
68 - Bob Estes, Roberto Castro, Boo Weekley, Russell Henley, Billy Mayfair, Nick O'Hern (AUS), Eric Meierdierks, Brad Fritsch (CAN), Joe Affrunti