Two members of the greenkeeping staff at Valhalla had been attempting to clear water from the putting surface with squeegees, but that proved an impossible task as the rain intensified.

The rain eased enough for tournament officials to announce play would restart after a delay of 45 minutes, although preferred lies could not be used as they were surprisingly not in force when the round got under way.

The delay meant McIlroy’s tee time was pushed back until 9:25am and by then the rain had stopped, leaving the course vulnerable to the kind of “target golf” the world number one prefers.

However, McIlroy’s opening drive on the 10th hole was pushed into the crowd and he was eventually unable to convert a birdie putt from 10 feet, although a par was at least two shots better than he managed there on Thursday after hooking his second shot out of bounds.

Another birdie chance went begging on the 11th and the Open champion then bogeyed the 12th, pulling his drive into the gallery and then failing to get up and down from a greenside bunker.

Playing partners Bubba Watson and Martin Kaymer – winners of the Masters and US Open respectively this season – needed to take drops from casual water on the 12th fairway but Watson was enjoying the conditions, making birdie on the 10th and 11th to move to three under.

At four under par McIlroy was three off the lead held by Palmer, who had followed six straight pars with a birdie on the par-five seventh.

World number four Henrik Stenson was just a shot off the lead after two birdies and one bogey in his first four holes, while Ryder Cup vice-captain Steve Stricker was alongside McIlroy after birdies on the 10th and 14th.