Payroll processing giant ADP has apparently rebuffed an overture from billionaire investor Bill Ackman, possibly setting the stage for a proxy fight.

Ackman has acquired an 8% stake in ADP through his Pershing Square Capital Management hedge fund, saying there is a “need for transformational change” at the company. But ADP announced Friday it had rejected his request to extend the Aug. 10 deadline for nominations to its board to facilitate negotiations.

“ADP is open to constructive input from our shareholders, and our Board respects the right of shareholders to nominate directors,” the company said in a news release. “However, ADP has a clearly defined board nomination process, and the 2017 deadline for director nominations has been public for nearly a year.”

“The board has unanimously determined that it is not in the best interests of ADP or its other shareholders to accede to Pershing Square’s last-minute request for an extension,” it added.

Ackman responded that he was “disappointed” by the board’s decision and he will now nominate a minority slate of directors. “Pershing Square believes that there is an enormous opportunity to improve the operating performance of ADP,” he said.

As Institutional Investor reports, rumors that Ackman was building a stake in ADP surfaced about 10 days ago, pushing the stock from under $100 to $120 before it dropped back down to close at $111.39 on Friday.

The hedge fund mogul has been seeking a big win since Pershing Square suffered heavy losses on investments in Valeant Pharmaceuticals International and Chipotle Mexican Grill. He previously invested in ADP from 2009 to 2011.

“For Pershing Square, this would be wading into known waters,” a person familiar with the hedge fund and its investment style told Fortune, adding “this is not Valeant.”

ADP noted Friday that since Carlos Rodriguez became its CEO nearly six years ago, “ADP’s total shareholder return of 202% is well in excess of the S&P 500 TSR of 128% — and is many multiples of Pershing’s TSR of 29%.”

“This year, however, it has underperformed the S&P 500 — even including the Ackman bump,” Institutional Investor said.

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