Mercedes team chairman Niki Lauda expects Williams to be the German squad's closest challenger when the season kicks off in Australia.
Williams has shown strong pace and even better reliability during winter testing and arrives in Melbourne as a challenger to Mercedes' supremacy following a competitive 2014 season.
While many expect Mercedes to dominate, Lauda reckons Williams will be a real threat.
"After the tests, I see the greatest challengers as the Williams team," Lauda told German broadcaster RTL.
"Valtteri Bottas is the biggest talent among the young drivers, but one should never underestimate Felipe Massa, who can be in front on a good day."
"With Sebastian Vettel and Kimi Raikkonen, Ferrari has a very good team and they will fighting against each other.
"Red Bull's top driver is clearly Daniel Ricciardo as I do not expect too much from Daniil Kvyat."