Adam Schenk has slender lead over big guns at Valspar Championship

The American signed for a hard-fought one under par 70 around the breezy Copperhead course in Florida, enough to move to eight under for the tournament, one ahead of the chasing Spieth and Fleetwood.

Walking onto the 16th tee, Schenk was one over for the round after a couple of bogeys and a birdie. But he rallied to birdie the 16th and 18th to keep his hopes of securing a maiden PGA Tour title alive.

Speith and Fleetwood both signed for 69s, but the former endured a tumultuous round, After opening with an eagle on the par five first, Speith rolled in seven pars as well as five birdies and bogeys for a colorful scorecard.

"I didn't have my best stuff in the approach game, but overall I'm in a good spot for tomorrow," the three-time major winner told pgatour.com.

"I made a few too many mistakes, but overall in these conditions, I think I would have signed for 2 under."

Meanwhile, Fleetwood was pleased with how he ground out gains in the challenging conditions, rolling in some pressure par putts to stay well in contention.

"I kept plugging away," said the Englishman.

"Pars were good. Birdies were hard to come by. The middle stretch of par saves on 9 and 10 were good putts to hole. I never went backward. I was very happy with anything par or better."

Further down the leaderboard, eight players are within three shots of the lead, including 2012 US Open winner Webb Simpson and Matt Wallace.