Spa-Francorchamps, Belgium - Formula One leader Nico Rosberg set the fastest time in opening Belgian Grand Prix practice on Friday after renewing his duel with Mercedes team mate and title rival Lewis Hamilton.
Returning refreshed from the August break, Rosberg set a best time of 1m51.577s around the classic Ardennes circuit.
Hamilton, who is 11 points adrift after 11 of 19 races, was a mere 0.097sec slower on a cloudy but bright morning at the longest track on the calendar which is renowned for fickle weather conditions.
The one-two was their fifth in succession in practice.
Ferrari's Fernando Alonso was third fastest with McLaren's Jenson Button fourth.
The Mercedes-powered teams were predicted to dominate and had seven cars in the top 10 but Ferrari's showing, with four times Spa winner Kimi Raikkonen fifth fastest despite a spin at La Source, indicated they would not have it all their own way.
Williams had a low-key start, with Finland's Valtteri Bottas 10th on the timesheets and Brazilian Felipe Massa 15th.
Red Bull's Daniel Ricciardo, winner of the previous race in Hungary, was ninth.
LOTTERER READY FOR F1
At the bottom end of the field, Germany's Andre Lotterer limbered up for his F1 debut with Caterham as a replacement for Japan's Kamui Kobayashi by lapping faster than Swedish rookie team mate Marcus Ericsson.
Lotterer may be twice as old as F1's latest teenage sensation Jos Verstappen but the 32-year-old intends to seize his opportunity this weekend when he takes over Kamui Kobayashi's Caterham seat at the Belgian Grand Prix.
"Sure, it came a little bit later than expected, but I cannot say I feel a worse driver at 32 than years ago!" said the three-time Le Mans winner.
"It's the opposite. I managed to get a lot of experience and feel much more confident over the years. Age is a number.
"I have been flat out racing all my life and I don't feel I have any problems with speed or any pains anywhere. I am taking the challenge."
Born in Germany, but brought up in Belgium, Lotterer has raced in the British and German F3 series before being named Jaguar's F1 test driver in 2002 on his way to his F1 race debut this Sunday.
He spent time in Japan in 2003 and went on to race in Formula Nippon and the Japanese Super GT Championship before he made his Le Mans 24 Hours debut in 2009, claiming overall victory in the race with Audi in 2011, 2012 and 2014.
He has also competed in the World Endurance Championship since 2012 and so arrives at Spa with the perfect package of experience to cope at one of his favourite circuits.
"I race Super Formula in Japan, very demanding and fast cars and I am still winning so why not?"
He added that he had sympathy for Kobayashi, but added "when I was younger I also had to watchother drivers take my seat or situations like this."
CHILTON BACK, ROSSI OUT
Max Chilton was restored in his race seat with the Marussia team on Friday and resumed as a driver in second free practice for the Belgian Grand Prix.
Marussia had announced on Thursday that American Alexander Rossi was taking over from the Briton due to contractual issues.
But the team revised its plans for Sunday's race, a move confirmed by the sport's ruling body, the International Motoring Federation.
Rossi took part in Friday morning's opening free practice session.
Chilton said that he had voluntarily stepped aside to allow the team to sell his seat to secure what he said was a much-needed financial boost.
A team spokesperson confirmed that Chilton would be racing and that an application had already been lodged with the FIA for him to be back in the car for Friday afternoon's second free practice session.
An FIA statement said: "The stewards have received a request from Marussia F1 Team to change the nominated driver Alexander Rossi (car #42) back to their previously nominated driver Max Chilton (car #4) for the 2014 Belgian Grand Prix.
"In accordance with Article 19 of the FIA Formula 1 sporting regulations the stewards grant permission for this driver change."
Marussia team chief Graeme Lowdon said: "These things happen. It is the driver's job to be flexible and both Max and Alex are professional drivers and will take this in their stride.
"It is a reflection of how the industry has to work sometimes." – Reuters/AFP
1 Nico Rosberg (Germany) Mercedes – 1m51.577s
2 Lewis Hamilton (Britain) Mercedes - 1:51.674
3 Fernando Alonso (Spain) Ferrari - 1:51.805
4 Jenson Button (Britain) McLaren - 1:52.404
5 Kimi Raikkonen (Finland) Ferrari - 1:52.818
6 Sergio Perez (Mexico)Force India - 1:52.903
7 Kevin Magnussen (Denmark) McLaren - 1:52.922
8 Nico Hulkenberg (Germany) Force India - 1:52.937
9 Daniel Ricciardo (Australia) Red Bull - 1:52.972
10 Valtteri Bottas (Finland) Williams - 1:53.172
11 Sebastian Vettel (Germany) Red Bull - 1:53.369
12 Daniil Kvyat (Russia) Toro Rosso - 1:53.594
13 Romain Grosjean (France) Lotus - 1:53.597
14 Adrian Sutil (Germany) Sauber - 1:53.703
15 Felipe Massa (Brazil) Williams - 1:53.968
16 Jean-Eric Vergne (France) Toro Rosso - 1:54.189
17 Giedo van der Garde (Netherlands) Sauber - 1:54.335
18 Pastor Maldonado (Venezuela) Lotus - 1:55.336
19 Jules Bianchi (France) Marussia - 1:55.782
20 Alexander Rossi (United States) Marussia - 1:57.232
21 Andre Lotterer (Germany) Caterham - 1:57.886
22 Marcus Ericsson (Sweden) Caterham - 1:57.977