Big rumors in Monaco have suggested the German has inked a two-year contract extension, but so far the Brackley team has said only that Rosberg and teammate Lewis Hamilton have "long term" deals.
"We have arrived at very favorable 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."
05/24/14 (GMM) Mercedes says it is making no further comment about Nico Rosberg's contract.
Amid reports the German has inked a further two-year extension to his already five-season stay at the Brackley team, a Mercedes spokesman said: "We have long term relationships with both our drivers."
A spokesman also told Die Welt newspaper: "The rumor of a contract extension has been around since the first race in Australia.
"We have not commented so far, and that will continue," he added.
Der Spiegel, meanwhile, said it made a similar enquiry to 28-year-old Rosberg's management, who reportedly 'did not deny' the contract extension news.
"They (Rosberg's management) did not want to comment further," Spiegel added.
Mercedes will hold a press conference in the Monaco paddock on Saturday morning, attended by title sponsor Petronas, Daimler chairman Dieter Zetsche, team boss Toto Wolff and drivers Rosberg and Lewis Hamilton.
Wolff told Der Tagesspiegel newspaper that "conspiracy theories" about Mercedes favoring championship leader Hamilton over Rosberg for the 2014 title are not true.
He insisted their contracts do not give either driver 'number 1' status.
"For us, they are both valuable, with their different personalities," said Wolff. "They both speak to our customers in different ways."
And he said the pair will continue to enjoy a free run to the title, unfettered by team orders.
"Of course, this includes all sorts of mind games and banana skins put in each other's paths, but this is ok so long as they don't throw them at the team," said Wolff.
05/23/14 (GMM) Mercedes is set to deal a knockout blow to speculation it might replace Nico Rosberg with Fernando Alonso in 2015.
When the Alonso rumors first emerged in Barcelona two weeks ago, team chairman Niki Lauda insisted Mercedes has "long term contracts" with both German Rosberg and Lewis Hamilton.
However, it was previously believed that Rosberg, 28, was contracted only until the end of this year.
But on Friday, the BBC reported that Rosberg's deal has been extended for at least two years.
"We have long term relationships with both our drivers," a Mercedes spokesman confirmed.