Michael Schumacher believes McLaren's poor form in testing suggests Renault and Williams will be Ferrari's main challengers this season.

The six-time drivers' champion has been unimpressed by the Mercedes-powered team's performances in the build-up to next month's Australian Grand Prix.

"I see Williams and Renault are really strong, probably more than McLaren," Schumacher said.

"But it's hard to judge as no-one knows what they are really doing at McLaren."

However, Schumacher told Italian newspaper Gazzetta dello Sport that McLaren may not be pulling out all the stops at the moment.

"McLaren aren't setting fast lap times, that's true, but maybe they aren't looking for it," he said.

"So it's pointless to talk about these things now, it's premature, we'll have to wait.

"Williams and us are certainly going better than others, but everything depends on the current tests and on the moment those lap times are set."