Delighted though he was by his best start in a major, Clarke said he knew the bunker-strewn course beside Lake Michigan would get tougher as the event progressed.
"Conditions were as easy as they'll get. I still think it is a par 77," he said.
Meanwhile, Vijay Singh, who like Els could topple Tiger Woods as world No1 by winning on Sunday, is on five under, alongside England's Luke Donald. Woods, however, suffered an error-strewn 75 in which he admitted he putted "atrociously".
Donald's performance could prove another step towards winning a wildcard Ryder Cup call-up from captain Bernhard Langer.
McGinley enhanced his chances of an automatic place with an opening round of 69 and Brian Davis was one shot further back.
Padraig Harrington, already certain of his team place, was on four under par but Colin Montgomerie could only manager a oneoverpar 73.
