Henrik Stenson was atop the early leaderboard at 8 under after a 4-under 68, with Davis Love III (69) one stroke behind.
First-round co-leader Chris Riley (72) held steady at 6-under, and Shaun Micheel (70) and Daniel Chopra were at 5-under. Woody Austin (69) and Adam Scott (69) were at 4-under with Sergio Garcia, who had to scramble after a triple-bogey on the par-4 11th.
Lucas Glover, who shared the lead with Riley on Thursday after showing just how badly he wants a spot on the U.S. Ryder Cup team, had an afternoon tee time. So did Tiger Woods and Phil Mickelson, playing together for the second day in a row.
That's if they can get on the course. Clouds moved in late in the morning, and the afternoon forecast called for rain.
Medinah has been redesigned since it hosted the PGA in 1999. At 7,561 yards, it's the longest in majors history and was supposed to be a beast. It has rolling hills and long rough, tricky greens and plenty of trees.
But that didn't stop 60 players going under par on Thursday — breaking by two the previous PGA Championship record for any round, set in 1995 in the second round at Riviera.
"I don't think there's anything wrong with it," said Stewart Cink, whose 68 Thursday left him two shots off the first-round lead.
"If somebody shoots 20-under par this week, then they are going to be the PGA champion, and they are going to have to go through a whole lot to win this tournament," Cink said. "It's a tough win no matter what."
Conditions were equally forgiving Friday. Stenson opened with back-to-back birdies and was at 9-under by the 13th hole. He gave that stroke back with a bogey on the par-4 15th, but rebounded with a birdie on the par-3 17th. But that stroke quickly disappeared as he closed his round with a bogey on the 18th.
Love, who needs a top finish to earn a spot on the Ryder Cup team, was all over the board — somewhat fitting considering the way he ended his first round, when he was tied for the lead before a triple-bogey on the 17th dropped him back into the pack.
He followed an opening birdie Friday with three straight bogeys, got back to even at the turn, and played the first six holes of his back nine at 3-under to jump into contention.
But the biggest move came from Chopra. The Swede, who shot even par Thursday, had four birdies on his front nine.
The course wasn't the only thing that didn't play out as expected. The pairing of the world's best players at the PGA Championship on Thursday wasn't exactly riveting stuff.
While unlikely suspects like Riley, Glover and Billy Andrade had the spotlight, Woods and Mickelson shot matching 3-under 69s.
Though Woods and Mickelson have won five of the last seven majors, they're rarely paired together. It's happened 15 times, only four of those at a major. They hadn't strolled around a major together, in fact, since the final round of the 2001 Masters — which, for those keeping score, Woods won.