Tiger Woods reflected on a topsy-turvy opening round at the US PGA Championship after a poor start and a disappointing finish left him well adrift of leader Brooks Koepka.

Having opted to rest rather than practice on Wednesday - a decision he confirmed was down to him "not feeling good" - Woods began his bid for victory at Bethpage Black with a double-bogey six.

This year's Masters champion reached the turn at three over before a run of two birdies and an eagle in four holes had him under par, raising hopes of closing the gap on in-form playing partner Koepka.

However, the 43-year-old faltered in the closing stages, three bogeys leaving him to sign for a two-over-par score of 72.

"It wasn't as clean as I'd like to have it for sure," Woods told the media.

"[I] didn't get off to a very good start. It was a good drive and ended up in a bad spot, and I compounded the problem with trying to use the backboard behind the hole there and missing a putt I should have made.

"And then I found my way back around, got it back under par for the day, and let a couple slip away with a couple [of] bad putts and a couple [of] mistakes at the end."

Koepka had no such issues on Thursday, the defending champion firing a brilliant bogey-free 63 to sit on seven under.

Asked about the possibility of catching his fellow American, Woods replied: "We'll see what the golf course offers up tomorrow.

"It changed quite a bit from when we played this morning to this afternoon. The greens got a lot faster. We'll see how it dries out over the next few days.

"I don't see them cutting the rough down, so it's just going to place another premium on driving the ball in the fairway to get at some of these flags."