"We have arrived at very favourable contract negotiations," team chairman Lauda told German television RTL.
"The last signature is missing, so it is not announced yet."
Lauda said the final signature will be added to the dotted line "some time soon".
"The driver pairing at Mercedes is perfect -- Nico as well as Lewis," he insisted.
"You can see it in the results that it is definitely not something that needs to be changed."
The door would therefore seem to be definitely shut to Fernando Alonso, despite Daimler chairman Dieter Zetsche causing a storm last week by saying the Spaniard is "perhaps the best" driver in F1.
"To be honest, I was in Spain with (King) Juan Carlos also present, and I am a polite man," Zetsche said in Monaco, explaining the comments.
"Alonso is definitely one of the best drivers, I think we can all agree," he added, "but what I also said - and it was not reported - is that we have a fantastic team and we are very happy."