Phil Mickelson knows he has no chance of winning the US PGA Championship if he continues to putt like he did in Thursday's opening round at Hazeltine.

The American world number two, among the later starters, had 34 putts in his two-over-par 74, finishing a humid day at Hazeltine National seven strokes behind leader Tiger Woods (four under).

Defending champion Padraig Harrington is second on three under.

"I had a terrible day putting, just terrible," left-hander Mickelson told reporters. "I haven't putted this bad in a long time. You cannot win golf tournaments putting like that."

Mickelson, who won the 2005 PGA Championship at Baltusrol, failed to capitalise on reaching 14 of 18 greens in regulation.

"I'm going to do something I haven't done in a long time, which is to go to the practice green after a round and spend some time. I've got to get this figured out before tomorrow."

The three-times major winner, who skipped last month's British Open to spend time with his wife Amy while she received breast cancer treatment, said he felt he could have been challenging for the lead had it not been for his putting woes.

"I feel like I'm throwing five, six, seven shots away on the greens," Mickelson said. "I don't feel like I'm playing as bad as I've been scoring. I felt it should have been . . . somewhere in the mid to high-60s today if I don't throw away those little three-footers here and there."

The 36-times PGA Tour champion played the four par-five holes a combined two under, but two bogeys and a double-bogey at the par-four 10th left him playing catch-up.