World champion Jorge Lorenzo led the way for Yamaha as MotoGP's final pre-season test started in Qatar.
In the first of three afternoon and evening sessions, Lorenzo was the first rider to lap beneath the 1m57s barrier, and then the first to lap under 1m56s, with a 1m55.962s lap set with just under three hours to go.
Having starred in the second test at Phillip Island last month, Suzuki's Maverick Vinales then went fastest with a 1m55.880s effort.
Lorenzo responded shortly afterwards with a 1m55.452s lap that would not be beaten.
With the circuit's race in just over a fortnight due to start at 9pm local time, and the test finishing at 11pm, riders were barely sighted in the final hour due to concerns about humidity and dropping temperatures making the track surface slippery.
The session was red flagged with just over one hour remaining to assess the conditions and clear debris when Aleix Espargaro fell.
Vinales finished 0.428 seconds behind Lorenzo, whose team-mate Valentino Rossi was third, despite falling at Turn 10 during his very first outlap of the day.
Andrea Iannone was fourth on his factory Ducati, ahead of Pramac satellite rider Scott Redding, who sat atop the times briefly, Hector Barbera and Andrea Dovizioso in a pack of Desmosedici GPs.
Cal Crutchlow finished the day ninth, ahead of Bradley Smith, while Marc Marquez was 10th for Honda, 1.071s off Lorenzo's pace, with team-mate Dani Pedrosa 13th.
Honda test rider Hiroshi Aoyama ran with winglets on the fairing of one of the manufacturer's RC213Vs for the first time, on his way to 21st.
Ducati's test rider, Michele Pirro is deputising for injured Pramac pilot Danilo Petrucci, and set the 17th-fastest time.
In the first public test for Aprilia's all-new RS-GP, Stefan Bradl was 19th and Alvaro Bautista 22nd, the former 2.614s off Lorenzo's ultimate pace.
Testing continues on Thursday and Friday, ahead of the season-opening Qatar Grand Prix on March 20.