Williams are prepared to release Valtteri Bottas to become Lewis Hamilton’s Mercedes team-mate in 2017.
Mercedes want the Finn, 27, to replace Nico Rosberg, who retired after winning the world title last month.
Deputy team principal Claire Williams told BBC Sport the team would let Bottas leave “if an experienced, credible alternative was available”.
Williams suggested Felipe Massa could return, despite retiring at the end of this season.
“I’m delighted to see that a team like Mercedes lists Valtteri as a potential replacement for Nico,” Williams said.
“We have always known Valtteri is one of the sport’s key talents and are proud that the championship leaders recognise this.
“However, Williams has its own ambitions and we must always ensure we give our team the best opportunity to move forward. Any changes would only be made if Williams remains in a strong position to compete and develop in 2017.
“If we did allow Valtteri to leave, we would only do this if an experienced, credible alternative was available, such as someone like Felipe Massa, for example.”
Williams, who finished fifth in the championship in 2016, have rejected Mercedes’ first offer for Bottas, but talks are ongoing.
The team are playing hardball with Mercedes F1 boss Toto Wolff because he is in a difficult position having to find a team-mate for Hamilton when all the leading names are under contract.
However, it is a sensitive situation because Williams use Mercedes engines and want to keep a good relationship with the German manufacturer.
And now Mercedes have expressed an interest, Bottas is keen to take up the opportunity to join a front-running team.
Mercedes said on Thursday that they would not make an announcement on Rosberg’s replacement until at least 3 January.
Bottas one of five options?
Three-time British world champion Hamilton will be consulted about German Rosberg’s replacement, but the final decision will be Mercedes’.
“Basically, once they’ve dwindled it down to five drivers – or whatever it is, the shortlist – contractually they have to discuss with me,” Hamilton, 31, told Channel 4.
“But I don’t have a say who it is.”
Massa to make Williams return?
Williams need an experienced driver for 2017 to partner the 18-year-old Canadian rookie Lance Stroll, who is making his debut and is said to be bringing financial backing worth as much as £20m.
Re-employing Massa could be a good solution for Williams. Although he announced his retirement from F1 in September, the Brazilian made it clear he wanted to continue racing. So a seat at Williams could be a good opportunity for him.
And it could provide a benefit for Williams because the team could use the money it saves on the engine fee and Bottas’ salary to fund technical developments and boost their competitiveness in future years.
Sources say Massa made his announcement only after he realised that Williams were going to keep Bottas and sign Stroll and that he was unable to find a similarly competitive car elsewhere.
Wolff’s first offer to Williams for Bottas was for a reduction of 10m euros (£8.4m) in their engine bill.
It is considered likely that Williams will hold out for a figure much closer to the full amount – 17m euros (£14.2m) – to release the driver.
They are aware that Mercedes have money available for the deal because they no longer have to fund Rosberg’s salary – which was set to be in the region of $22m (£17.6m) for each of the next two years.
In a separate deal, Mercedes executive director (technical) Paddy Lowe is poised to join Williams in a senior management role.
Mercedes are set to replace him with the former Ferrari technical director James Allison.