Lewis Hamilton and Jenson Button both fear Red Bull Racing have something up their sleeve that will see them again clinch pole for tomorrow's Turkish Grand Prix.
Hamilton finished quickest at the end of yesterday's first practice and McLaren team-mate Button topped the timesheet with the best lap of the day come the conclusion of the second 90-minute session.
The duo sandwiched the Red Bull pairing of man-of-the-moment Mark Webber and Sebastian Vettel in the afternoon, but there is genuine concern their rivals were sandbagging.
"I have no doubts that in qualifying the Red Bulls will pull out something extra, which they always seem able to do," said Hamilton.
"I think they'll be very quick, along with the Ferraris, although I do feel we will be able to compete with them.
"We're always fast in practice, but they always go a little faster in qualifying, and I expect that today."
Button, whose time of one minute 28.280 seconds was marginally quicker than last year's pole lap from Vettel, echoed Hamilton's comments.
"My car is good, but I don't think the Red Bulls are showing their hand yet, so don't get too excited about our pace," said Button.
"The Red Bull is still the car to beat, and we will see where we stand.
"We're positive people and so let's hope we can challenge them, but it's going to be difficult."
Webber, who has won the last two grands prix at a canter in Spain and Monaco, finished within 0.098sec of Button.
However, in the dying moments of second practice the 33-year-old Australian was forced to pull over with an issue to his Renault engine.
"The engine went, but it was on high-mileage so we were expecting it to be on the edge," said Webber.
Vettel experimented for the first time with Red Bull's own version of the 'f-duct', a device pioneered this season by McLaren that increases straight-line speed by reducing drag.
The team have yet to make a decision whether they will continue to use it this weekend as Vettel said: "We still need to do some fine tuning, but it looks good.
"McLaren look very competitive, and they seem quite a bit quicker than everyone else on the straights.
"You can make up time in the corners, but there are quite a few straights, so we'll see if we keep our system on."
Hamilton, meanwhile, has been cleared by the FIA to race with the sleepers he is sporting in both ears following a recent visit to the United States.
FIA regulations prohibit "the wearing of jewellery in the form of body piercing or metal neck chains."
However, when Hamilton eventually wears the diamond studs he is planning to buy, an FIA spokesperson has confirmed that may be more closely monitored.