Formula One's sparring championship protagonists Lewis Hamilton and Nico Rosberg again proved there is little to separate them.
In-form Hamilton emerged top dog from Rosberg at the end of Friday's two practice sessions for the United States Grand Prix by a hair's breadth, just three thousandths of a second.
Hamilton had topped FP1 at the Circuit of the Americas by a clear 0.293secs from Rosberg, with McLaren's Jenson Button their closest challenger.
But in switching from the medium-compound tyres used in FP1 to the soft Pirelli rubber given an outing in FP2, Rosberg managed to reel in his Mercedes team-mate.
Hamilton's day also ended under a slight cloud as his car encountered a hydraulic problem with 20 minutes remaining, bringing an early end to his day's running.
It underlined the fact that despite winning the last four grands prix to open up a 17-point cushion to Rosberg, Hamilton's hopes of claiming a second title rest as much on the reliability of his car over the final three races as his own talent.
The duo, though, were head and shoulders above any of their rivals, with Ferrari's Fernando Alonso the best of the rest, and not for the first time this season.
Alonso finished a staggering 1.104secs adrift of Hamilton, such is the performance advantage of the Mercedes at this track.
Red Bull's Daniel Ricciardo, who only completed five laps in FP1 due to his car encountering an ERS (energy recovery system) problem, was back on track in FP2.
Ricciardo rattled off 29 laps to finish fourth quickest, whilst in contrast team-mate Sebastian Vettel propped up the timesheet.
It was confirmed in between the two sessions the team had installed a complete power unit, and with it being the sixth of the season, the outgoing four-times champion serves a penalty that will see him start from the pit lane.
However, Vettel almost immediately encountered problems with the new system as he managed only 19 laps, finishing nearly five seconds off the pace.
Significantly, Vettel was last of only 18 cars - the financial problems encountered by Caterham and Marussia over the past week forcing both teams into administration.
With both teams and out of this event, it means the field for this weekend's race will be the lowest since the 2005 Monaco Grand Prix.
Behind the leading quartet was Williams' Felipe Massa, followed by Kimi Raikkonen in his Ferrari and Toro Rosso's Daniil Kvyat.
McLaren duo Kevin Magnussen and Button were eighth and ninth respectively, the latter 1.613secs adrift, with Force India's Nico Hulkenberg completing the top 10.
Significantly, in the dying stages of the second session the FIA tested their new virtual safety car (VSC) system.
The device has been introduced in the wake of Jules Bianchi's life-threatening crash in the Japanese Grand Prix nearly four weeks ago.
Bianchi went off the Suzuka circuit under double-waved yellow-flag conditions, forcing the FIA to experiment with the VSC - which forces a driver to slow significantly via a signal sent to the cockpit of his car.Reuse content