Muller and Lopez traded times for the majority of the hour-long session on the famous track, with Frenchman Muller eventually setting a fastest time of 8m39.950s after the session time had expired to usurp Honda's Rob Huff.
Lopez found more time shortly afterwards to split the pair.
Huff improved his time with five minutes to go - setting what at the time had been the fastest second, third and fourth sectors - to jump to the head of the leaderboard with a 8m42.220s.
But Muller blitzed the Briton's effort a couple of minutes later, going a full 2.270 seconds quicker in his Citroen C-Elysee before Lopez also bumped Huff down.
Tiago Monteiro was fourth behind Honda team-mate Huff, 2.292s off the Briton's pace, while Tom Chilton was fifth in his Sebastien Loeb Racing C-Elysee and the fastest independent driver.
The third factory Honda Civic of Norbert Michelisz and Lada's Hugo Valente separated Chilton from team-mate Mehdi Bennani, the Moroccan ending the session eighth-fastest having spent much of the hour in the top five.
Thed Bjork was ninth fastest, with both he and Polestar team-mate Fredrik Ekblom opting to focus on the set-up of their Volvo S60s during the session.
Nicky Catsburg was 10th ahead of Lada team-mate Gabriele Tarquini, while Munnich Motorsport stand-in and Nurburgring specialist Sabine Schmitz was 12th.
Tom Coronel picked up a puncture around a quarter of the way through just his second lap and was 13th.
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