Brooks Koepka believes he is starting to read the Shinnecock Hills greens better after surging into contention at the U.S. Open.

The defending champion had fallen to seven over early in his second round, but six birdies in his last 11 holes helped him match the low round of the tournament with a four-under 66 to get to one over.

"I felt like I played really well. It was unfortunate, just hit a few bad drives. And if you do that out here, it's going to penalise you pretty bad," Koepka said after his round.

"I felt like I was playing solid. My iron play was pretty good. Putting – the putts were going in. I felt like I hit good putts yesterday. Nothing went in. Where today, they were kind of bouncing in. Obviously, it's poa, so they're going to be a little bouncy.

"But it was nice to see some go in, and I think I've got these greens a little more figured out today."

Five shots back entering the third round, Koepka believes he is in great shape to potentially repeat as champion.

"You don't want to be that many back. But it's a U.S. Open, so disaster's always around the corner," he said.

"You never know what's going to happen. You just keep putting the ball in play, hitting greens and try to sneak in a few birdies when you can."

Englishman Ian Poulter knows all about the potential disasters after he endured one on his 17th hole. Poulter was just one shot behind leader Dustin Johnson at three under, when he suffered a triple bogey-bogey finish. 

While Koepka could catch Johnson with a couple of solid rounds, he thinks the world number one will have to give some shots away in order to be caught.

"There's not many birdies. Like I said, there's a disaster around every corner," Koepka said.

"I mean, all it takes is one shot in the fescue, and you could be in there for a while. But, obviously, you need a good round tomorrow just to give yourself a chance. You know, anything within three shots of the lead on the back nine Sunday, anything can happen."