According to a statement filed with the SEC, ADP said Ackman owns 8 percent of ADP, largely through derivatives. He asked the company to extend the deadline for nominations of directors from the current August 10 for 30 to 45 days and said he plans to nominate five directors, including himself to ADP's 10-member board.

Pershing has three nominees and cannot assume control unless the deadline is extended.

ADP also defended the record of Rodriguez, who became CEO in November 2011. It said total shareholder return of 202 percent in that time is significantly higher than that 128 percent of the S&P 500 in that time, and well above Pershing's rate of 29 percent. It also said the company has an independence board, with four new members have joined since 2014.