By Alan Baldwin

SHANGHAI (Reuters) - The usual suspects will end up fighting for this season's Formula One championship and Ferrari can be contenders despite struggling with their car's performance, according to Fernando Alonso.

The double world champion is leading the standings in his Ferrari after two of 20 races following a victory against the odds in rain-hit Malaysia last month.

However there have been surprises with Sauber finishing second thanks to Mexican Sergio Perez in Malaysia, and having Japan's Kamui Kobayashi lining up in third place on the grid for Sunday's race in China.

Williams and Lotus, previously Renault, have also looked competitive.

"It's true that at the moment we see some surprises, a very quick Williams in Australia, Sauber is doing a fantastic job in this part of the world championship," Alonso told reporters after qualifying ninth.

"But I guess to fight for the world championship only the top teams will be there (in the end) because they have the facilities, they have the budget, the experience of fighting for the championship.

"So I guess McLaren, Mercedes and hopefully Ferrari will be at the end challenging (Red Bull) for the world championship. The other teams, the resources and capacity they have are maybe more limited compared to the top teams."

Ferrari have struggled to extract performance from their step-nosed car and Alonso said the Sauber, which also has a Ferrari engine, was quicker.

"Obviously we are not stupid, and we know we are quite far behind and we need to work," he declared. "One second is a lot of gap to recover but the car has some big problems in terms of aerodynamics."

Alonso, who won his two crowns with Renault, said the problems also provided scope for optimism because a lot of time could be recovered by putting a couple of things right.

"This is the first priority. What is not working now, try to make it work and when we are happy with the car we will see what is the gap exactly," he said. "At the moment it is too big because the car is not working."

Ferrari technical director Pat Fry told reporters earlier that the team needed to work on a range of areas from the way the wind tunnel was run, its accuracy and simulation to making parts lighter and more durable.

"There's work going on absolutely everywhere within the company, on the basic fundamental methodology as well as just trying to upgrade the car," he explained.

(Reporting by Alan Baldwin, editing by Martyn Herman)