Suzuki rider Andrea Iannone topped the opening day of MotoGP testing in Jerez, edging out Cal Crutchlow by less than a tenth of a second.
This week's Jerez test, which runs from Wednesday to Friday, is being attended by the majority of MotoGP teams - with only Honda, Yamaha, Tech3 and Aspar absent - plus a number of leading World and British Superbike squads.
The Pramac Ducati squad did not take part on Wednesday, but is set to join the action on Thursday.
Iannone hit the top of the times shortly before midday, knocking Kawasaki WSBK rider Tom Sykes off the top of the times with a 1m38.652s.
That stood as the benchmark until factory Ducati rider Jorge Lorenzo posted a 1m38.483s with a little under three hours remaining, although that was soon beaten by his team-mate Andrea Dovizioso, who set a 1m38.398s.
Iannone retook top spot with a little over an hour remaining, setting a 1m38.280s that put him within 0.031 seconds of Dani Pedrosa's pole time from this year's Jerez race, which was held before the track was resurfaced.
Crutchlow moved up to second aboard the LCR Honda on a 1m38.337s before a late crash for Ducati WSBK rider Chaz Davies at Turn 1 caused a lengthy late stoppage with 30 minutes to go.
Dovizioso's time was good enough for third, ahead of Lorenzo and leading KTM rider Pol Espargaro, who also caused a red flag when he crashed at Turn 3 in the third hour.
Sykes's early benchmark meant he was the fastest WSBK representative in sixth overall ahead of Avintia Ducati's Tito Rabat and his Kawasaki team-mate Jonathan Rea.
Bradley Smith (KTM) and Scott Redding (Aprilia) rounded out the top 10.
Sylvain Guintoli was in action for Suzuki, ending up 15th-fastest, while Eugene Laverty - standing in for Aleix Espargaro at Aprilia - was one place behind in 16th.
Moto2 frontrunner Alex Marquez had his first taste of the Marc VDS Honda a day ahead of schedule, but ended the day down in 25th place, some 3.4s off the pace.