The Red Bull driver posted a 1:40.403 around the Valencia street circuit, 0.824s up on Petrov while Ferrari's Fernando Alonso was a further 0.012s off the pace.Lewis Hamilton was fourth quickest, ahead of Nick Heidfeld and Felipe Massa.
As for reigning World Champion Sebastian Vettel, he was down in 16th place, 2.538s off the pace.
Reports claim the German's lack of pace was due to him running without the off-throttle exhaust, which will be banned from the next race in Britain.
Report: Track temperature was 28'C as Friday's first practice got underway in Valencia with Sebastian Vettel leading out the field for their installation laps. Nico Rosberg set the first lap time of the session, a 2:06.412 for the Mercedes GP driver. Force India reserve driver Nico Hulkenberg joined the action with a 1:47.942.
He continued lapping, improving his time before his session came to a premature end as the German lost the rear end of his VJM04 - actually Paul di Resta's - under braking, hitting the concrete barriers at Turn 12.
30 minutes in the the session only six drivers, Hulkenberg, Pastor Maldonado, Jaime Alguersuari, Daniel Ricciardo, Michael Schumacher and Nico Rosberg, had set lap times while the others waited for the street circuit to be cleaned and grip to improve.
Maldonado continued running, taking the P1 slot with a 1:43.455. Schumacher overhauled him with a 1:43.050 as Ricciardo took second place. Adrian Sutil jumped to fourth while the two Ferraris were eighth and ninth with Fernando Alonso ahead of Felipe Massa. Both were over a second off the pace.
Massa was the next to lead the way with a 1:42.530 while Nick Heidfeld put his Renault up into second place before he lost out to Sutil. Alonso took over from Massa at the top, 0.847s up on his team-mate's best time. Lewis Hamilton went sixth on his first run with Jenson Button, who won the previous grand prix in Canada, P7.
Problems for Karun Chandhok, who is touring slowly in his Lotus and reporting a problem with his second gear. The Lotus test driver, who is taking Jarno Trulli's drive for this session, headed back into the pits. Sutil climbed to second place, splitting the two Ferraris. Mark Webber joined the action, taking sixth place. The Red Bull driver improved to second on his next lap. Button climbed to fourth.
Webber took over at the top, setting the fastest first and third sector time, on his way to a 1:40.597. His team-mate Vettel was down in 18th place, over three seconds off the pace, with reports claiming he is running without the off-throttle exhaust as it will be banned from the next race in Britain.
With 30 minutes remaining, Webber was leading the way ahead of Alonso, Massa, Button, Sutil, Hamilton, Heidfeld and Rosberg. Problems for Vitaly Petrov, who Renault report has burnt his back in his overheating R30.
Webber upped his pace, shaving tenths off his time while Petrov was finally back in the car, moving up from 11th to fourth place. The Russian took over six-tenths off his time to climb to second place behind Webber and ahead of Alonso. Hamilton put in a better lap, taking fourth place. However, the McLaren driver was still over a second off the pace.
Practice One ended with Webber in the P1 slot with a 1:40.403, 0.824s ahead of Petrov and a further 0.012 up on Alonso.
Times
01. Mark Webber Red Bull-Renault 1m40.403 22 laps
02. Vitaly Petrov Renault 1m41.227 + 0.824 20 laps
03. Fernando Alonso Ferrari 1m41.239 + 0.836 22 laps
04. Lewis Hamilton McLaren-Mercedes 1m41.510 + 1.107 23 laps
05. Nick Heidfeld Renault 1m41.580 + 1.177 24 laps
06. Felipe Massa Ferrari 1m41.758 + 1.355 23 laps
07. Jenson Button McLaren-Mercedes 1m41.926 + 1.523 14 laps
08. Adrian Sutil Force India-Mercedes 1m41.955 + 1.552 20 laps
09. Nico Rosberg Mercedes 1m42.043 + 1.640 22 laps
10. Jaime Alguersuari Toro Rosso-Ferrari 1m42.216 + 1.813 29 laps
11. Michael Schumacher Mercedes 1m42.270 + 1.867 26 laps
12. Sebastien Buemi Toro Rosso-Ferrari 1m42.412 + 2.009 27 laps
13. Rubens Barrichello Williams-Cosworth 1m42.704 + 2.301 23 laps
14. Sergio Perez Sauber-Ferrari 1m42.738 + 2.335 20 laps
15. Pastor Maldonado Williams-Cosworth 1m42.841 + 2.438 28 laps
16. Sebastian Vettel Red Bull-Renault 1m42.941 + 2.538 21 laps
17. Kamui Kobayashi Sauber-Ferrari 1m43.201 + 2.798 18 laps
18. Nico Hulkenberg Force India-Mercedes 1m43.769 + 3.366 7 laps
19. Heikki Kovalainen Lotus-Renault 1m44.136 + 3.733 17 laps
20. Jerome D'Ambrosio Virgin-Cosworth 1m45.026 + 4.623 17 laps
21. Timo Glock Virgin-Cosworth 1m45.221 + 4.818 19 laps
22. Tonio Liuzzi HRT-Cosworth 1m45.494 + 5.091 24 laps
23. Narain Karthikeyan HRT-Cosworth 1m46.926 + 6.523 27 laps
24. Karun Chandhok Lotus-Renault 2 laps
Source: Planet-F1
