AUSTIN, Texas (AP) The Mercedes duel is hitting full throttle at the U.S. Grand Prix.
Formula One championship leader Nico Rosberg put more pressure on teammate and rival Lewis Hamilton by posting the fastest lap in Friday's practice sessions as he seeks to tighten his grip on a potential first title.
Hamilton led the morning session before Rosberg came back in the afternoon with a lap of 1 minute, 37.358 seconds. Hamilton was third in the afternoon session, but his morning lap of 1:37.428 was second-quickest on the day.
''I wasn't comfortable in the second session,'' Hamilton said. ''We made some (braking) changes ... we were struggling to finish a lap. We'll be fine tomorrow.''
Hamilton trails Rosberg by 33 points heading into the final four races, starting Sunday.
Early speed seldom is a predictor for the finish line in Texas.
Rosberg started from the pole each of the last two years only to see Hamilton win. Hamilton's 2015 victory clinched his second consecutive Formula One championship.
If Rosberg can pick up a win in any of the final four races, he will join Michael Schumacher, Sebastian Vettel and Hamilton as the only drivers with 10 victories in a single season. And if Rosberg wins Sunday, it puts him within range of the single-season record of 13 victories shared by Schumacher (2004) and Vettel (2013).
Hamilton, who has won three times in four years at the Circuit of the Americas, has called the track ''good hunting ground'' for him. He needs to rack up wins down the stretch to catch his teammate.
But Rosberg can win the season title with three second-place finishes and a third even if Hamilton ends the season on a tear. Even so, Rosberg has said he won't settle for the safety of second place and will push Hamilton for the win.
The team lurking behind Mercedes is Red Bull.
Daniel Ricciardo, who finished third here in 2014 and briefly led last season's rain-soaked race, was third-fastest Friday with hopes of gaining more time in Saturday's qualifying. His Red Bull car blasted through the long straights and showed power Mercedes may not have expected in their rear-view mirror.
''I'll take that but I'll prefer pole (position) and a win. I don't know what Mercedes is doing, but I feel like we have more pace to get. Our long runs looked the best,'' Ricciardo said. ''I thought it could have been better, but maybe Nico and Lewis say the same.''