Steve Stricker posted a record-equalling lowest round to lead in major history after the first day of the US PGA.
The American hit a seven-under 63 with countryman Jerry Kelly two shots behind the 44-year-old.
US Open winner Rory McIlroy finished on a level-par 70, despite injuring his wrist on the third hole when he hit a tree stump with his club. He will have an MRI scan to determine whether he can compete on Friday.
Meanwhile, Tiger Woods hit a seven-over 77 in his worst opening round in major history.