The offer period for Comcast's £14.75 per share bid to buy 61% of Sky will now remain open for acceptances until September 12.
Comcast, which owns NBCUniversal, gatecrashed Fox's attempt to buy Sky earlier this year.
Sky’s board had originally backed Fox’s offer of £14.0 per share but withdrew its support after Comcast increased its bid in July.
For the Fox offer, which values Sky at £24.5bn, Sky shareholders have until September 17 to accept.
Rupert Murdoch’s Fox has until September 22 to revise its current offer for Sky, which represents a 5% discount to Comcast's current offer.
Fox first launched its bid to buy the rest of the share in Sky it does not already own in December 2016 but the acquisition was held up by UK regulators over concerns about giving the Murdoch family too much power and influence over British media.
Comcast confirmed a takeover proposal for Sky in April.
Separately, Fox has agreed to sell its entertainment assets to Walt Disney Co (NYSE:DIS) for US$71bn. Comcast had tabled a US$65bn deal to buy Fox in June but pulled out of competing with Disney a month later to focus its efforts on buying Sky.