Brooks Koepka insisted he was not yet at his best despite moving into the Tour Championship lead at the halfway mark.

The American world number one carded a three-under 67 in the second round at East Lake to push into a one-stroke lead on Friday.

Koepka, who birdied the final hole to take the outright lead, said he was getting close to his best.

"I feel better. I don't feel like I'm clicking 100 per cent, but it's definitely close," he told reporters after his round.

"There are shots where it's like, 'how did I just do that?' And there are some shots where I'm like, 'man, why can't I do that every time?' Like on 18.

"I feel good. I feel very confident. I've got no problem where I'm at. I feel very confident with the putter, which is most important."

Play was delayed for 90 minutes during the second round due to inclement weather before Koepka finished the day at 13 under – a shot ahead of Justin Thomas and Rory McIlroy.

Koepka was one of only five players to manage a round of 67 or better and the American was satisfied with his performance.

"The lead's always nice, so I'll take that. I played good today. I putted really good. Short game was pretty solid," he said.

"The rain delay kind of killed any momentum I had. I didn't feel like I had any good golf shots after the rain delay, but that's part of golf.

"Everybody's got to deal with the same thing, just didn't execute."