Yet after the summer break, it is Rosberg who started a winning street that culminated in his dominant performance in Singapore, where he recaptured the lead of the world championship with just six races now to go.
"Nico had a fantastic weekend," quadruple world champion Prost told French broadcaster Canal Plus.
"On Saturday he got pole and on Sunday he was in full control of the race. For Hamilton it was not such a good weekend - not competitive and less consistent.
"Before Singapore, it seemed that Hamilton was more competitive but last weekend Nico proved that he is a very strong opponent."
However, Prost thinks that it will continue to be a close battle for the title over the remaining six races of the year.
"We have seen with Mercedes that one driver seems to perform very well and the other a little less. We have not really seen them battle wheel to wheel but it seems to me that this is likely to happen before the end of the season," he said.
"For me, the outcome of the title will depend on the details, the psychology, the effectiveness of the starts and possibly other cars interfering with their fight."
The championship continues in Malaysia next weekend.