Jenson Button denied Rubens Barrichello the perfect practice performance by setting the fastest Formula One lap seen at the Indianapolis Motor Speedway.
Barrichello had topped the time sheets after each of the first three practice runs, but the Brazilian was forced to settle for fourth in the final session as Button came within a fraction of cracking 70 seconds for the 4.192km circuit.
The 24-year-old, pushing Barrichello for second place in the drivers' standings, was on the pace in his BAR as he produced a pulsating lap of one minute 10.056 seconds.
Qualifying this afternoon for tomorrow's United States Grand Prix appears as if it will be a duel between BAR and Ferrari as they occupied the top four positions with times all quicker than the best posted by Barrichello at the end of yesterday's practice sessions.
Michael Schumacher was just 0.143secs slower than Button, whose team-mate Takuma Sato avoided any incidents this morning - following his collision with the Sauber's Felipe Massa yesterday - was third, just under 0.2secs adrift.
It left Barrichello fourth despite setting his best time in practice with 1:10.351, with Juan Pablo Montoya fifth in his BMW Williams, and Fernando Alonso, who had lost a bargeboard off his Renault in the third run after hitting a kerb too hard, in sixth.
The top 10 was completed by Cristiano da Matta (Toyota), Ralf Schumacher (BMW Williams), Jarno Trulli (Renault) and last year's pole Kimi Raikkonen (McLaren), with team-mate David Coulthard 12th and more than 1.3secs down.
Mark Webber was the only casualty in the final 45-minute session, losing the Jaguar on approach to turn six due to what appeared to be a mechanical fault and sliding across the gravel before nudging the nose into a tyre wall to finish 15th.