Mercedes executive director Toto Wolff says that talks are ongoing with regards to Williams using Mercedes engines from 2014.
Williams is set to switch to using Mercedes power in 2014 following dissatisfaction with the cost of Renault's engines.
Williams has been powered by Renault since the start of 2012 but is under no contractual obligation to continue with the French squad into next season.
"It is clear we are losing an engine customer in 2015 and for Mercedes it is important to have a regular business case," said Wolff.
McLaren will switch to Honda power from the 2015 season. Apart from their own team, Mercedes will also continue to supply Force India.
"So it is important to have three customers from 2015 onwards and that is why we are having certain discussions with a number of teams, where Williams is one of them," he added.
Wolff also said that any deal with Williams would be from 2014, rather than 2015.
"No, it would be for 2014. You can't sign a one-year deal with anyone," he said.