
The offer was previously set to expire at midnight on 28 September, but has been extended until midnight on 13 October.
HP is understood to have tabled an offer of US$4.5bn for Mercury, which is involved in business technology optimisation (BTO) software and services.
BTO products help customers govern and manage IT, and optimise application quality, performance and availability.
HP is currently embroiled in a boardroom spying scandal over alleged illegal wiretapping that has already claimed a handful of the company's top brass.
Shortly before HP's former chairman Patricia Dunn and chief executive Mark Hurd testified before a congressional committee, the company disclosed that its general counsel Ann Baskins had jumped ship.
After resigning Baskins declined to testify in front of the panel, citing her Fifth Amendment right not to give testimony that could be self-incriminating.
Although Dunn and Hurd expressed regrets in their testimonies, neither have admitted direct responsibility for the tactics that led to the HP investigation and congressional hearing.