Ben Spies moved ahead of Ducati riders Nicky Hayden and Valentino Rossi on his final lap of a damp opening practice session for the Portuguese MotoGP at Estoril.

Practice took place under sunny skies, but with damp patches of asphalt - and a few puddles through the fast final turn - following heavy overnight rain, repeating the 'in-between' track conditions in practice last weekend at Jerez.

Just a handful of riders - including Yamaha star Spies - went out with wet tyres in the first half of the 45-minute session.

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Wets were exchanged for soft option slicks as further rider made an appearance during the second half of the session, culminating in best time of 1m 40.813s lap by Spies.

That put the Texan 0.273s ahead of countryman Hayden, but well off the race lap record of 1m 36.937s set by Dani Pedrosa in 2009. Spies, looking to turnaround a poor start to 2012, recorded 17 laps.

Rossi (+0.689s) was the final rider within one-second of Spies during his eight laps. The struggling seven time MotoGP champion intends to continue work on a Hayden-style set-up for his Ducati this weekend.

Reigning world champion Casey Stoner (+1.130s) rode for six laps on his way to fourth for Repsol Honda.

Yamaha's world championship leader Jorge Lorenzo - celebrating his 25th birthday today - and fellow Spaniard Hector Barbera (Pramac) chose not to record a lap time due to the conditions.

Second practice takes place this afternoon, but changeable weather is forecast for the remainder of the weekend.

Estoril is rumoured to be dropping off the MotoGP calendar after this year's event.

Friday practice (1):

1. Ben Spies
2. Nicky Hayden
3. Valentino Rossi
4. Casey Stoner
5. Alvaro Bautista
6. Cal Crutchlow
7. Andrea Dovizioso
8. Dani Pedrosa
9. Yonny Hernandez
10. Karel Abraham
11. Randy De Puniet
12. Michele Pirro
13. Stefan Bradl
14. Colin Edwards
15. Danilo Petrucci
16. Aleix Espargaro
17. Mattia Pasini
18. Ivan Silva
19. James Ellison