NEW YORK (Reuters) - The CEO of Automatic Data Processing Inc ADP.O slammed investor William Ackman on Thursday, likening the hedge fund manager to a “spoiled brat,” and skewering the billionaire’s research efforts into the payroll processor.

William 'Bill' Ackman, CEO and Portfolio Manager of Pershing Square Capital Management, speaks during the Sohn Investment Conference in New York City, U.S., May 8, 2017. REUTERS/Brendan McDermid

ADP CEO Carlos Rodriguez, in an interview with CNBC, called his interactions with Ackman “baffling and surreal.” Rodriguez took several pot shots at the investor, saying Ackman was relying on disgruntled employees for research and that he prioritized a vacation over a meeting with ADP’s board.

The interview was a rare display of a CEO bashing one his company’s largest investors and set the stage for a proxy fight as the two sides try to keep each other’s nominees off ADP’s board.

Last week, Ackman’s Pershing Square Capital Management disclosed an 8 percent holding in the $50 billion U.S. human resources outsourcing company, and nominated three directors to the board. Ackman, one of the nominees, wanted ADP to extend its board director nomination deadline, which expires on Thursday. The board refused.

Rodriguez told CNBC Ackman sought more time because the hedge fund manager was unprepared.

“It kind of reminds me a little bit of a spoiled brat in school asking a teacher for an extension on their homework,” the CEO said in the interview.

Pershing Square declined to comment on Rodriguez’s comments to CNBC, saving its response for a conference call it previously planned on Aug. 17 to discuss its ADP investment.

Rodriguez said Ackman originally told him he wanted the CEO to be replaced, a view Pershing has since changed.

At one point, Rodriguez said Ackman “does not know what he’s talking about” and was relying on disgruntled ex-ADP employees for his information. Still, the CEO said he was willing to listen to Ackman if the investor had ideas to boost shareholder value. The two plan to meet in early September, he said.

ADP, a global company with 630,000 customers and based in Paterson, New Jersey, usually holds its annual shareholder meeting in early November.

Rodriguez noted on Thursday that for 42 years, the company has increased its dividend and “if Bill Ackman leaves us alone, we intend to get to 50.”