World champion Michael Schumacher is becoming wary of Williams after initially identifying McLaren as Ferrari's main Formula One rivals this season.

The German told a news conference on Thursday that Williams appeared to have made important advances in recent testing and he expects them to be increasingly involved in the title battle.

"I have seen a big step forward from Williams at the last test in Barcelona," the Ferrari driver said before Sunday's Brazilian Grand Prix at Interlagos.

Schumacher's younger brother Ralf topped the times in Barcelona testing.

"As we saw early in the championship, the points are not clearly given to one driver at the moment. It is I think open between the two teams (to be Ferrari's main rivals)," said Schumacher.

"After this race or during this race I think Williams will have a much better opportunity. If that is the case, then they will come into the championship as contenders as well and it will be between three teams rather than two to fight for the championship."

McLaren have won the first two races of the season with Briton David Coulthard triumphant in Australia and Finland's Kimi Raikkonen taking his first career win in Malaysia.

Colombian Juan Pablo Montoya, on pole in Brazil last year and leader the year before, was second for Williams in Australia but well out of the points in Malaysia after being involved in a first lap shunt.

The Colombian, who treats Brazil as a local race, said earlier this week that he would be surprised if Ferrari won at Interlagos on Sunday.

Schumacher, winner of a record 11 of 17 races last year, has yet to stand on the podium in 2003.