Valentino Rossi has compared MotoGP rivals Jorge Lorenzo and Casey Stoner to a pair of circling sharks - ready to strike at the first sign of weakness by the Italian.

The reigning six-time MotoGP world champion has seen his title cut from 30 to 18 points after Lorenzo took his fourth win of the year last Sunday in Estoril, a race that also saw Stoner claim a strong second on his comeback from fatigue problems.

Rossi, meanwhile, managed only fourth.

"If I am not strong, I know that Casey and Jorge will eat me in one bite," AAP reports Rossi as saying.

But Rossi believes he successfully repelled one 'shark attack' by the pair earlier in the season, after being beaten by both Stoner and Lorenzo in his home Italian Grand Prix, and fully intends to stay on top.

"Earlier this season I was not very strong, not fast enough... In this case, Stoner and Lorenzo are the sharks and they looked at me with a little bit of blood flowing and maybe they thought, 'OK, now is the time'.

"They both finished ahead of me at Mugello in May [where Rossi had been unbeaten since 2002] and they thought now we have beaten Valentino in Italy we can beat him everywhere.

"But fortunately I became stronger."

Rossi has won six races so far this season. Three races and 75 points remain, starting with the Australian Grand Prix at Phillip Island on October 18.