Rosberg has won eight races to Hamilton's six, including the last three, and heads into Sunday's Malaysian Grand Prix with an eight-point advantage.
No driver has won eight times in a season and failed to take the world championship but Rosberg remained behind triple champion Hamilton in the betting odds.
BoyleSports had Hamilton at 8/13 for the title with Rosberg on 5/4. The nearest rival was Red Bull's Australian Daniel Ricciardo at 500/1.
Ladbrokes and paddypower.com offered Hamilton at 4/6 and Rosberg at 6/5.
"It is incredibly tight at the top of the drivers’ championship and while Rosberg has the advantage on paper, we think Lewis Hamilton is still favourite but only just," William Hill spokesman Joe Crilly said after Rosberg won in Singapore.
(Reporting by Alan Baldwin, editing by Ed Osmond)
