Tiger Woods concedes he faces an increasingly tough task to match Jack Nicklaus's record tally of 18 major titles after seeing his chances of a fifth US PGA Championship victory disappear.

Woods began the third round eight shots off the lead but could only card a second consecutive 72 - which included birdies on the 16th and 18th - at Harding Park to slip to two over par.

The 44-year-old ended an 11-year wait for his 15th major title in unforgettable fashion in last year's Masters, but knows time is running out due to his advancing years and problematic fused back.

"There's not as many (majors left) as when I first started playing," Woods said. "The reality is that the golf courses are getting bigger. They are getting longer.

"The margin between making the cut and the lead is a lot smaller than it used to be. Used to be sometimes 12 to 15 shots. Now, we had, what, nine shots? It's just different.

"It's getting tighter and it's getting harder to win events, but you look at the leaderboard of most major
championships, you see the same guys. May not be always the same winners, but you see the same handful of guys are there.

"They understand how to win major championships, how to win the big events, how to plod their way along, how difficult it is to win these big events."

Woods started his third round with seven straight pars before three-putting the eighth and also bogeyed the 11th, 12th and 13th before a brace of late birdies put a more flattering gloss on his score.

"It's just like yesterday," Woods added. "I just didn't get anything going and had to claw and fight to get my way back and didn't really get anything going until the last few holes.

"Happy the way I finished. I made some putts on the last five holes, but that's about it. It was just a fight all day."