Soaking conditions greeted riders at the start of Assen’s first 45 -minute training weekend, with forecasts looking unsatisfactory for three days.
Despite the miserable conditions, the track was a hive of activity early on as most headed outside to get wet under their belts.
Suzuki’s Mir set an initial speed of 1m50.988s, but he was the first to show how difficult conditions were when he ran under braking for Turn 1.
Turn 1 will be an incident point for the first half of the session, as Miller and Maverick Vinales (Aprilia) of Ducati crossed under the braking there and ran, while both Enea Bastianini of Gresini and Marco Bezzecchi of VR46 crashed perpendicular to the anchors.
Both riders were OK, but deteriorating conditions forced most half the distance back to the pitlane.
At this point Miller – who got his first MotoGP win in Assen in 2016 in wet conditions – sat at the top of the timesheets with 1m44.523s.
Johann Zarco of Pramac was his closest challenger in the second, but it wasn’t until the last 10 minutes when Miller’s time was finally better as the rain eased and track conditions improved.
Francesco Bagnaia-who admitted on Thursday in Assen that he still had no answer to his costly crash at the Sachsenring last week-replaced teammate Miller with 1m44.368s with less than nine minutes left.
Vinales knocked him out a minute later by 1m44.128s, before Pol Espargaro at the Honda factory and Miller dueled for top honors as the session ended.
Espargaro was the first to break through the 1m42s barrier with 1m42.891s with two minutes left, but Miller finished the session fastest of all by 1m42.589s.
Mir, the 2020 world champion, outscored Miller to the checkered flag by 0.109 seconds, with Pol Espargaro completing the top three.
Alex Marquez-who will leave LCR Honda at the end of 2023-is fourth fastest ahead of Aleix Espargaro of Aprilia and Miguel Oliveira of KTM, who won the wet Indonesian GP in March.
Alex Rins – who will soon move to LCR to replace Alex Marquez – is seventh ahead of Pramac Ducati duo Zarco and Jorge Martin, with Vinales entering the top 10.
Bagnaia is 11th on the checkered flag ahead of the fastest of rookies Fabio Di Giannantonio (Gresini), who scored more valuable wet track time in his 2021-spec Ducati.
Brad Binder was 13th ahead of Bezzecchi following his early crash, with championship leader Fabio Quartararo 17th at his factory Yamaha who sat in the top six in an FP1 stage.
Bastianini was late after his Turn 1 spill, whose grid grew to 25 riders this weekend while Aprilia’s Lorenzo Savadori made another wildcard appearance this year. He is 22nd.
FP2 for the MotoGP Dutch GP will start at 2:10 pm local time.