The CEO of Automatic Data Processing Inc. 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.

ADP CEO Carlos Rodriguez pulled no punches in an interview with CNBC, calling his interactions with Ackman "baffling and surreal."

Ackman's Pershing Square Capital Management disclosed an 8 percent stake in the $50 billion U.S. human resources outsourcing company last week, and nominated three directors to serve on the board. Ackman is one of the nominees.

Ackman built his 8 percent stake in the company largely through derivatives, rather than common stock, according to ADP.

Rodriguez said tensions between the company and the investor escalated when Ackman asked ADP to extend its director nomination deadline, which the board refused. On several different occasions, Rodriguez said, Ackman asked for extensions ranging from one week to 45 days.

"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.

Rodriguez's candor in the interview was a rare case of a CEO bashing a major investor and offered a clear sign that the two sides are headed for a full blown proxy fight.

But Rodriguez also said he was "willing to listen" to the investor and was in the process of scheduling a meeting with him in early September. Still, Rodriguez said, Ackman has not offered any ideas on how to boost shareholder value yet, and has relied on research that is shoddy.

A Pershing Square spokesman declined to comment on the interview.