Calciomercato.com says Mertens is believed to favour remaining at Napoli, but the 33-year-old has not put pen to paper on a new deal due to a number of key disputes with the club.
The forward is demanding a two-year deal worth €4million per season while he is also hunting for a renewal bonus.
Additionally, Mertens also wants Napoli to rescind the fines imposed on their players following an internal mutiny against the club hierarchy last November.
However, club chairman Aurelio De Laurentis does not wish to cancel the financial penalties while he is also reluctant to pay more than €3m per season in wages.
Chelsea are in the driving seat if a renewal at the Stadio San Paolo cannot be agreed, and it is acknowledged that a move to the Premier League would be tempting.