'Step by step they are coming to the front' - Gerhard Berger.
The famous Austrian, who formerly co-owned the Toro Rosso team, was asked whether Briton Hamilton's arrival at Mercedes is the ultimate test for German Rosberg.
"That's what they said about (Rosberg alongside Michael) Schumacher," Berger, referring to Rosberg's former teammate, smiled to Germany's Auto Motor und Sport.
"Rosberg has proved how good he is. If he can also hold Hamilton at bay, it will be another milestone.
"For me, those two (Rosberg and Hamilton) are on the same level. Each has his own strengths.
"One has the cooler head, the other more emotion, and it's pretty even," he added.
Berger, 53, also said he has not been surprised by Mercedes' clear step forward in 2013.
"It's not surprising," he insisted, "because they have the best engine and two great drivers.
"It was clear that at some point they would start to get the chassis right.
"Step by step they are coming to the front," added Berger.