Brooks Koepka made an ominous start to his bid for an historic third straight victory in the US PGA Championship as Tiger Woods kept himself firmly in the frame on a top-class leaderboard in San Francisco.

Koepka carded an opening four-under-par 66 at Harding Park to lie a shot off the early clubhouse lead held by former world number one Jason Day, the Australian carding five birdies in a flawless 65.

Former Ryder Cup team-mates Justin Rose and Martin Kaymer were alongside Koepka on four under, with Open champion Shane Lowry two strokes further back following a 68 matched by Woods.

The 15-time major winner was two under par after four holes, fell back to level par and then made three birdies in four holes before making a bogey on his penultimate hole at a venue where he won a WGC event in 2005 and compiled a 5-0 record in the 2009 Presidents Cup.

Starting on the back nine at Harding Park, Koepka failed to take advantage of the par-five 10th and dropped a shot on the next, but then birdied the 13th, 14th, 16th and 18th thanks to some superb approach play.

A bogey on the first slowed Koepka's momentum, although he did well to limit the damage to a single shot after only moving his second shot 40 yards from the rough and then finding a bunker with his third.

Koepka, who had managed just one top-10 finish in nine starts either side of the Covid-19 shutdown before last week's tie for second in Memphis, bounced back with birdies on the second and fourth and declared himself satisfied with his opening effort.

"I mean, it's only 18 holes right now," Koepka said. "I feel good. I feel confident. I'm excited for the next three days.

"I think I can definitely play a lot better, and just need to tidy a few things up, and we'll be there come Sunday on the back nine."

Woods, who is using a slightly longer putter this week to ease the strain on his troublesome back, said: "I figured we were going to get the more favourable conditions today, (but) some of these pins were on the difficult side.

"This course is all about hitting fairways, if you are able to hit the fairways you can get after some of those pins."

Playing partners Rory McIlroy and Justin Thomas had to settle for rounds of level par and one over respectively, McIlroy battling back from a hat-trick of bogeys on his front nine with three birdies in four holes around the turn.

McIlroy won the WGC-Cadillac Match Play at Harding Park in 2015 but came into the first major of the year having recorded a best finish of 11th in his five events since the PGA Tour returned to action following the Covid-19 shutdown.

Kaymer has not won since the second of his two major victories in the 2014 US Open and revealed he watched a replay of the back nine of the final round on YouTube on Wednesday evening.

"Definitely that video from last night me winning the US Open, that helped me to believe that my putting was good enough; that my ball-striking was good enough, even though it's a few years back, but it's always nice to remember those moments and feel the same that you felt that day."

Koepka, who also won the 2010 US PGA at Whistling Straits, added: "I know you shouldn't live in the past, but if the past can help you in the present moment, I'll take it."