Coulthard wary of Williams

Ralf Schumacher and Juan Pablo Montoya, pole sitter for three of the last four races, were expected to set the pace at the Brickyard thanks to the superior power of their BMW V10.

However, the Williams duo trailed half a second behind Mika Hakkinen's pace-setting time in ninth and 10th places respectively as one second separated the top 15 cars.

Coulthard believes that despite a lacklustre first day, Schumacher Jr and Montoya could yet be a force this weekend and should not be underestimated.

"We're reasonably happy with the way the car was going given the fuel level we're running," said Coulthard. "But the big thing, of course, is qualifying, and most notably this year we've seen Williams take a big step forward on Saturdays."

The Scot is also wary that Ferrari could come back to lead the way in qualifying, despite team mate Hakkinen setting the pace on Friday.

"We've seen Ferrari be very strong all season in qualifying," he said. "Tomorrow [Saturday] we'll find out if we're more connected than at Monza."

Coulthard set fastest lap in last year's race but by that time it was all too late as his chances of victory had been scuppered by penalty for jumping the start. Even so he battled back to finish fifth as Ferrari romped to a 1-2 triumph.

The 30-year-old has been critical of Indianapolis in the build up to this weekend, but was more complimentary of the Indiana state capital on Friday and he believes having a US GP is essential to the F1 World Championship.

"It's nice to be back in America," he said. "And in order to have a world championship we need a race in the USA. We have arrived in the aftermath of some terrible events, which have taken some of the edge of it, but we are here to win."

For combined practice results, click here.