Despite Merc downplaying their Jerez performance, the W05 has caught the eye of their rivals with Adrian Sutil saying it "looks quite strong."
Although it is undoubtedly too early to make any predictions after just one pre-season test, Mercedes emerged the strongest after four days of running at Jerez.
Clocking 309 laps, they covered more mileage than any other team, while Lewis Hamilton's 1:23.952 set last Wednesday was the week's third fastest time.
Asked by Sky Sports News for his thoughts on the early running, new Sauber signing Sutil said: "For me it's too early to say who is quick.
"I think the Mercedes package looks quite strong. They do a lot of laps and I have a feeling they are in the front right now. But let's see."
As for his own team's performance, the 31-year-old revealed that Sauber had not been pushing for pace as mileage was more important.
"Our approach was probably different to other teams here. It was more like a rollout, I wasn't really driving on the limit because it was too early for that.
"The car is still not consistent enough to really tweak it to the limit. The lack of downforce is also very tricky to drive, the conditions were not great.
"We didn't want to take any risk and we had no [spare] parts here so it was most important to keep the car in one piece and it kind of worked.
"So we finished the whole four days and now a lot of data to study."
The German also confirmed that the C33 he drove at Jerez would be very different to the car used at the next outing at Bahrain as Sauber are facing a "strong development curve."
"Especially for us from here going to Bahrain it will be a huge step to the front," he added."
"Here we were just limited to what we could do but I expect a lot of changes and hopefully good changes for the first part of the season
"It will be quite a strong development curve I think, especially thinking about the new rules. That should be normal."