Marquez fastest in rain-hit Japanese MotoGP practice
The Spanish great has only recently returned from a long injury absence but he showed all his experience to master the conditions at Motegi for a leading time of one minute and 55.232 seconds on his Honda.
That was ahead of Ducati riders Jorge Martin, also from Spain, and Jack Miller.
Australia's Miller topped a mostly dry first practice on Friday and still holds the fastest lap, 1:44.509, so far.
The forecasted rain duly arrived on Saturday, when there is qualifying scheduled for later in the day, and the trio of title contenders had to settle for places further down the timesheets.
World champion and current leader Fabio Quartararo was 10th on his Yamaha in the wet second practice, a place ahead of Aprilia's Aleix Espargaro.
Ducati's Francesco Bagnaia, who has closed Quartararo's championship lead to 10 points with fives races left, was 15th.
The Italian was second-fastest on Friday in the dry.
The wild weather is forecast to dump heavy rain on the Motegi area on Saturday but should clear up for Sunday's race.
Japan is hosting its first MotoGP since 2019 because of the pandemic.
Leading times from the first two practice sessions for the Japanese Grand Prix:
1. Jack Miller (AUS/Ducati) 1min 44.509sec, 2. Francesco Bagnaia (ITA/Ducati) at 0.028sec, 3. Fabio Quartararo (FRA/Yamaha) 0.049, 4. Aleix Espargaro (ESP/Aprilia) 0.068, 5. Luca Marini (ITA/Ducati-VR46) 0.136, 6. Marc Marquez (ESP/Honda) 0.147, 7. Pol Espargaro (ESP/Honda) 0.169, 8. Brad Binder (AFS/KTM) 0.226, 9. Miguel Oliveira (POR/KTM) 0.234, 10. Maverick Vinales (ESP/Aprilia) 0.288