ACI Worldwide Inc. has extended its exchange offer for shares of S1 Corp. until Oct. 31. The offer, which has a blended value of $9.44 per share, was set to expire September 28.

ACI and S1's board have been involved in a pitched battle for control of S1 since July, when ACI launched its hostile bid for the rival vendor.

The bid attempted to derail a merger S1, of Norcross, Ga., had planned with Fundtech Ltd. That merger, which fell apart for other reasons, would have created an end-to-end payments company that specialized in corporate treasury and trade finance.

Fundtech instead agreed to be acquired by the private-equity firm GTCR, which in turn plans to merge Fundtech with BankServ, a payments software company GTCR bough tin August.

As of Tuesday, ACI said 12,965,097 shares of S1 common stock had been tendered in the exchange offer.

In conjunction with its extension, ACI, which is based in New York, withdrew its previous bid under Hart-Scott-Rodino Antitrust Improvements Act, but will file it again Wednesday.