Defending Turkish Grand Prix champion Kimi Raikkonen had the fastest lap in the first practice yesterday for this year's Formula One race.
The Finnish McLaren-Mercedes driver, who won the inaugural race last year from pole position, timed 1 minute, 28.315 seconds on the 5.32km circuit.
In the afternoon practice, BMW-Sauber test driver Sebastian Vettel was fastest in 1:28.091.
Vettel once won 18 times in 20 races in the German BMW championship in 2004.
He was hired after Sauber dropped Jacques Villeneuve two weeks ago and Robert Kubica became a race driver.
There is another practice this morning before qualifying this afternoon. Tomorrow's race is over 58 laps.
However, the focus was on Michael Schumacher and Fernando Alonso as they continued their duel for the F1 title.
Schumacher went out late in his Ferrari and briefly led with 1:28.777 before Raikkonen's drive.
Alonso did not go out in that session as Renault prefers to conserve the engine and tires on its cars.
In the afternoon, Schumacher was sixth in 1:28.819, while Alonso was 12th in 1:29.741.
Honda's Jenson Button, who won the Hungarian GP three weeks ago for his first victory in nearly seven years of F1 racing, was third in the morning in 1:28.785 and third in the afternoon in 1:28.506.
With five races left in the season, Alonso has seen his lead cut from 25 points to 10 in the last four races with Schumacher winning three.
Alonso, who won six of the first nine races this season, has since finished second, fifth twice, and failed to complete the Hungarian GP after a wheel-nut broke off while he was comfortably leading.
With a victory worth 10 points, Schumacher can share the lead if he wins and Alonso is scoreless.
The Istanbul Park circuit is unique with rollercoaster-like hills and a counterclockwise direction.
Last year, more than 200,000 people attended the race weekend, causing massive traffic jams on the roads to the circuit, which is almost 80km from the center of Istanbul.
Local officials said they would try to alleviate the problem this year by extending parking capacity to 30,000 and allocating four roads for private vehicles traveling to the race, as well as a fifth to serve public transport.