McIlroy and Horschel set to cross swords again
Horschel carded a superb 67 to share the clubhouse lead in the 113th US Open at Merion with compatriot Phil Mickelson, but McIlroy's rounds of 73 and 70 were enough to leave him just four shots off the pace.
He and Horschel fell out at the Walker Cup in 2007, with the world number two turning on the American when the pair met on the final day at Royal County Down.
"I was up against Horschel on the first day, too," McIlroy recalled the following year.
"I messed up the last hole and lost to him. I wasn't happy.
"Then I got him again, twice, on the Sunday. His antics were really p****** me off. For example, he had hit a bunker shot at the 14th in our morning foursome. It was a great shot and finished inches from the cup. But he came running down the hill hollering at the top of his voice. He was so loud and so obnoxious.
"Anyway, that outburst of his was probably the worst thing he could have done. In the second singles, I set out to be as loud as he was.
"On the first tee I ripped a drive up the middle, hit my 7-iron approach to maybe 15 feet - I was shouting 'Be good! Be good!' all the way - and holed the putt for an eagle.
"When the ball went in, I gave it the loudest 'Come on!' you've ever heard. I think he got the point."
Those differences appeared to be a thing of the past when the pair played a practice round alongside Tiger Woods earlier this week, with Woods also alongside McIlroy on three over after matching rounds of 73 and 70.
The knock-on effect of Thursday's weather delays meant the second round had not been completed on Friday evening, with Horschel and Mickelson sharing the lead ahead of English pair Luke Donald and Justin Rose and veteran American Steve Stricker.