Formula One championship leader Lewis Hamilton posted the fastest time in Friday's two practice sessions at the U.S. Grand Prix, but abandoned the second session with about 10 minutes to go to deal with a fluid leak on the cockpit floor.
Afterward, Hamilton seemed confident the problem would be resolved before Saturday qualifying.
"We stopped and pulled the floor up and there was some kind of fluid ... either a fuel leak or an oil leak," Hamilton said. "I'm not thinking about it. We just lost a little time in the session."
Hamilton's fastest lap at the Circuit of the Americas was 1 minute, 39.085 seconds in the afternoon. His Mercedes teammate and title rival Nico Rosberg was .003 seconds behind.
Hamilton is chasing his second world championship. He won the title in 2008 with McLaren and has a 17-point lead over Rosberg with 100 still available in the final three races. The final race in Abu Dhabi will be worth double points, a new system that Hamilton this week questioned as an unfair way to wrap a season-long dual between himself and Rosberg.
Racing against Rosberg is challenging enough. The last thing Hamilton wants to deal with at this late stage in the season is a reliability problem that could cost him points. The only races Hamilton didn't finish on the podium this season were the three he didn't finish.
Rosberg seemed to sense an opportunity.
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"He had a major problem," Rosberg said, noting the pressure that continues to mount within the Mercedes team.
"The tension is there for sure," he said. "It's a great battle. That's what I enjoy about racing such a strong teammate. You're always searching for that little bit extra. It's going to be three great races coming up."
Defending race champion Sebastian Vettel's long season took another expected blow when the FIA ruled he will have to start Sunday's race from pit lane because he will have to use a new, sixth power unit in his engine, breaking the limit of five.
Vettel had acknowledged the possibility earlier in the week and the FIA confirmed the penalty after Vettel used the new power unit in Friday's first practice. While Vettel had hinted he might skip qualifying, Red Bull has said he likely will drive the first session but may pit the car after that to save wear and tear.
Ferrari's Fernando Alonso had the day's third-fastest time at 1:40.189.