Championship leader Lewis Hamilton maintained his supremacy in Friday's second free practice session for this weekend's United States Grand Prix when he outpaced his Mercedes teammate Nico Rosberg by just 0.003 of a second.
But Hamilton finished his day back in the pits as his team worked on his car to deal with an unspecified leak that raised reliability alarms as he pursues his fifth straight win.
Hamilton had also been fastest in the morning session and his afternoon supremacy meant that he had now been fastest in 14 of the 17 second free practice sessions this year.
The two Mercedes men, separated by just 17 points in the title race, were more than a second clear of third-placed man, two-time champion Fernando Alonso of Ferrari.
"The team just said to me 'you've got to stop, there's something wrong with the car. But I don't think it particularly affects the weekend," Hamilton said.
"They pulled the floor off and there was some kind of fluid, some kind of leak. I had to stop just in case. The gearbox was working fine and the race box goes in tomorrow and I am confident that will be fine."
Australian Daniel Ricciardo, winner of three races for Red Bull this year, was fourth ahead of Felipe Massa of Williams and Kimi Rakkonen in the second Ferrari.
Russian Daniil Kvyat was seventh for Toro Rosso ahead of Dane Kevin Magnussen and his McLaren team-mate Briton Jenson Button and 10th placed German Nico Hulkenberg.
Last year's winner, four time champion Sebastian Vettel, who is leaving Red Bull at the end of the season, was only able to complete a few installation laps at the end of the session following a gearbox change.
He will start Sunday's race from the pit lane after changing his power unit.
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