Porsche's Andre Lotterer set the quickest time on the opening day of free practice for the Fuji round of the World Endurance Championship.
Lotterer, driving the #1 Porsche 919 Hybrid, turned in a time of 1:25.527s during the first of two 90-minute runs that were each affected by adverse weather.
Toyota led the second session through Jose Maria Lopez, but the Japanese driver could only muster a 1:39.202s lap as conditions worsened around the 2.8-mile circuit.
That meant the times from the morning counted as the top figures of the day, with Porsche recording a one-two as Earl Bamber backed up Lotterer's benchmark in the championship-leading 919.
Lopez finished four tenths off the pace in the #7 TS050 Hybrid, marginally ahead of the sister Toyota driven by Sebastien Buemi.
Victory at Fuji would secure Porsche's third consecutive WEC manufacturers' title, while Bamber, Brendon Hartley and Timo Bernhard only need to beat the Buemi/Anthony Davidson/Kazuki Nakajima Toyota to clinch the drivers' crown with two rounds to spare.
In LMP2, Jackie Chan DC Racing topped the timesheets in both sessions.
Series leaders Thomas Laurent, Oliver Jarvis and Ho-Pin Tung had their championship lead reduced to 20 points with finishes outside the top three in Mexico and the United States, but were back on top in FP2 at Fuji.
Thomas Laurent clocked in at 1:47.437s during the afternoon, although that time fell five seconds short of the marker set earlier in the day by Alex Brundle, behind the wheel of the sister ORECA 07.
TDS Racing set the second-quickest time of the day courtesy of Mathieu Vaxiviere.
The French team's ORECA was then involved in an incident during second practice, when former grand prix driver Vitaly Petrov spun into the barriers to bring out the day's only red flag.
As in the top category, Porsche's factory squad provided the best two times in GTE-Pro.
Michael Christensen recorded a 1:47.365s run during FP1, supported by a 1:47.617 from Richard Lietz in the other 911 RSR.
The Porsches were edged out in second practice, however, as Olivier Pla set the quickest lap in his Ford GT.
DTM expat Miguel Molina led the way in GTE-Am, driving the Spirit of Race Ferrari 488.
Qualifying for round six of the World Endurance Championship is scheduled to take place on Saturday, October 14th at 14:00 local time (06:00 UK).
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