McLaren's Jenson Button just beat Red Bull's Mark Webber to the fastest time in practice at the Turkish Grand Prix.
The world champion's lap of one minute 28.280 seconds was just 0.098secs ahead of the Australian, whose team-mate Sebastian Vettel was third fastest.
Webber, the world championship leader, suffered a suspected engine failure towards the end of the session.
McLaren's Lewis Hamilton led after the first session, but ended fourth while Ferrari's Fernando Alonso was fifth.
Alonso completed a congested top five, just half-a-second behind Button, prompting Ferrari's chief engineer Chris Dyer to tweet via Twitter: "All in all a reasonable session for us. I believe we can be competitive both in qualifying and in the race."
And Button said he, too, was optimistic of a strong race on Sunday, although he warned that challenging Red Bull would be difficult.
"Our car works well round here," said the 30-year-old.
"There are a few small things I want to sort out, but I'm reasonably happy with the balance and both tyres are working well.
"But we are still trying to chase Red Bull, who are yet to show their hand here, so don't get too excited by our pace.
"It was a normal Friday and important to get as much information and learn about the car because you don't catch up one second over a couple of weeks.
"The Red Bulls are still the cars to beat and we've got to hope we can get everything together in qualifying
"On Saturday we'll see Red Bull's true pace. We've got to hope for something better, let's hope we can challenge them, but it's going to be difficult."
Nico Rosberg outperformed his Mercedes team-mate Michael Schumacher as the Germans finished sixth and seventh.
A number of drivers had problems at the triple-apex Turn Eight during the two 90-minute free practice sessions.
All the drivers struggled to cope with debris and dust on the racing line in the morning session, but as speeds increased in the afternoon practice, so too did the spin-offs.
Force India's Adrian Sutil walked away from a crash into the barriers at the end of session one but with his car patched up, returned to complete 16 laps in the afternoon and post the 11th quickest time.
Ferrari's Felipe Massa, who has won three of the five Formula 1 races held in Turkey, also left the track at the notoriously difficult corner on three occasions.
The third exit was the most costly for the Brazilian who destroyed one of the softer option tyres he was using in a bid to post a quick time and he had to settle for 10th fastest overall.
BMW Sauber's Kamui Kobayashi, Renault's Vitaly Petrov and Force India's Vitantonio Liuzzi were also among those caught out by the big left-hander.
Red Bull duo Webber and Vettel experienced few problems through that section of the track as they both clipped more than one second off their morning times to post 1:28.378 and 1:28.590.
However, with the session reaching its climax, Webber's car stopped as it was exiting Turn Two and the Australian was forced to freewheel backwards off the track with wisps of smoke coming out of the engine cover.
Red Bull are not too concerned about the incident, however, as they say the engine was nearing the end of its life.
"Mark's engine was right up at the end of its mileage," said team principal Christian Horner.
"I think it stopped about 50 kilometres short of its target miles, so it doesn't affect his programme."
As Webber's car trundled back to safety, he was involved in a heated exchange with the marshals as the Australian tried to get out of the vehicle.
"I was telling them, just find some common sense," said Webber.
"There is no handbrake on an F1 car, so I was telling them to put something behind the wheel to stop the car rolling back.
"It took them 10 minutes to try and explain that I couldn't stop the car from rolling back. It was a simple thing but frustrating."
The Red Bulls, who were expected to dominate, failed to match the speed of Button though.
The Englishman also eclipsed his morning time by more than one second to move above compatriot Hamilton, who, unusually, posted a slower time in the afternoon.