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Phil Mickelson's dream week at the US PGA Championship continued Saturday, putting him in a position to become golf's oldest major championship winner.  The 50-year-old survived early on the back nine to shoot 70 and hold a one-stroke lead over Brooks Koepka at seven-under par for the tournament.

He knows he does not have many chances left to collect his sixth major title, but he said he's trying to keep his focus on his game rather than thinking about the big-picture implications.

"I think that because I feel or believe that I'm playing really well and I have an opportunity to contend for a major championship on Sunday and I'm having so much fun that it's easier to stay in the present and not get ahead of myself," Mickelson told reporters.

Mickelson opened a gap on the field early on moving day with birdies on four of his first seven holes, then another at the 10th.

"I felt I had a very clear picture on every shot, and I've been swinging the club well, and so I was executing," he said of that stretch. "I just need to keep that picture a few more times."

The picture got a bit fuzzy at 12 and 13, where Mickelson went bogey-double bogey.

He said his focus slipped on those two holes.

"It's just an example of losing the feel and the picture of the shot, and I get a little bit jumpy, a little bit fast from the top, and it just -- when that happens I get narrow and I end up flipping it," he said.

"So those two swings were more a product of not staying or keeping the feel and the focus of the shot. And so that's just a small little thing that I need to iron out."

Mickelson finished the round with five consecutive pars to ensure his place atop the leaderboard heading into Sunday.

His remarkable week has put him in a position to surpass Julius Boros, who won the 1968 PGA at age 48, as the oldest major winner, but Mickelson was in no mood to ponder what he already had accomplished just by getting this far.

"I'm more focused on a few things that I need to work on tonight before tomorrow's round, and I'm not really dwelling back on what took place today," he said.

"I just know I'm having a lot of fun and I'm very appreciative of the way the people have been supportive."