Reigning MotoGP champion Jorge Lorenzo turned up the heat on Casey Stoner by taking over at the top during Friday afternoon's practice session for the US MotoGP at Laguna Seca.

The warmer afternoon temperatures gave the expanded 18-rider field a taste of what track conditions to expect in Sunday's race, and saw Lorenzo dominate from start to finish - dispute a scare in the final minute - while Stoner struggle to improve on his morning time.

Lorenzo, who has beaten Stoner at the last two rounds and is now 15 points from the title leader, sliced 0.3sec from the Australian's FP1 time to finish the day with a 0.141sec advantage over Stoner's Repsol Honda team-mate Dani Pedrosa.

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Stoner finally beat his morning time on his final lap to finish 0.175sec from Lorenzo in third, with Andrea Dovizioso making it three Repsol RCVs in a row, in fourth (+0.491sec).

Lorenzo's Factory Yamaha team-mate Ben Spies was again the top American, in fifth (+0.559sec), with San Carlo Honda Gresini's Marco Simoncelli sixth (+0.747sec) and Suzuki's Alvaro Bautista seventh (+1.000sec).

Ducati's confusing season continued as Pramac's Randy de Puniet jumped ahead of both factory Desmosedici riders to claim eighth place (+1.079sec), with Monster Yamaha tech 3's Colin Edwards in ninth (+1.127sec).

Then came factory Ducati riders Nicky Hayden and Valentino Rossi, who slipped back from seventh and ninth respectively this morning, as the gap to the top also slightly increased.

Rossi looks set to stick with the GP11.1, while Hayden had tried both the GP11 and - for the first time - the GP11.1 this morning.

Tech 3's Cal Crutchlow was again the top Laguna rookie in 13th place, moving a tenth closer to the front, but with fellow rookie Karel Abraham (Cardion AB) just behind him.

37-year-old Ben Bostrom completed his first ever MotoGP laps, on a second LCR Honda, on Friday.

The American - who currently races for Michael Jordan Motorsports in the AMA Superbike Championship, where he will continue to compete this weekend - was 4.986sec from the top this morning and 4.103sec this afternoon.

Bostrom finished 1.496sec behind Pramac's Loris Capirossi, who is returning from injury this weekend.

The Factory Yamaha team are riding in special red-and-white livery for the second time this season, continuing their 50th anniversary of grand prix road racing celebrations. Spies took his first ever MotoGP win in red-and-white at Assen.

Friday practice (2):

1. Lorenzo
2. Pedrosa
3. Stoner
4. Dovizioso
5. Spies
6. Simoncelli
7. Bautista
8. de Puniet
9. Edwards
10. Hayden
11. Rossi
12. Barbera
13. Crutchlow
14. Abraham
15. Elias
16. Aoyama
17. Capirossi
18. Bostrom