The runaway championship leader, also quickest in Friday's pre-event test, went top after the first flyers and his Chevrolet was never displaced thereafter.
With 10 minutes to run he underlined his dominance by slashing almost one second from his previous benchmark and dropping to a 1m49.096s, which was more than eight tenths faster than anyone else had managed.
That lap came just moments after home favourite Jose Maria Lopez, making his WTCC debut with the Wiechers BMW crew, had moved into second and within 0.005s of Muller's previous best.
The three-time Super TC2000 champion's time proved good enough to land him fourth overall, and was almost one second faster than any other BMW driver.
He was only bumped from second in the final minutes, as first Honda's Tiago Monteiro and then Muller's RML Chevrolet team-mate Tom Chilton improved.
Both men were still more than six tenths adrift of Muller however.
Monteiro was also forced to take an early engine change, meaning he will start from the back of the grid for race one regardless of qualifying.
Behind the top four, Zengo's Norbert Michelisz beat James Thompson in the lead Lada to fifth, while a late improvement helped Gabriele Tarquini to seal seventh.
Hugo Valente was the top SEAT man in eighth, fractions ahead of defending champion Rob Huff, while Alex MacDowall rounded out the top 10.
The 30-minute session was initially delayed due to technical issues with the television broadcast, but ran without incident once the problem was resolved.
Pos Driver Team/Car Time Gap
1. Yvan Muller RML Chevrolet 1m49.096s
2. Tom Chilton RML Chevrolet 1m49.749s +0.653s
3. Tiago Monteiro Honda 1m49.861s +0.765s
4. Jose Maria Lopez Wiechers BMW 1m49.906s +0.810s
5. Norbert Michelisz Zengo Honda 1m50.082s +0.986s
6. James Thompson Lada 1m50.304s +1.208s
7. Gabriele Tarquini Honda 1m50.413s +1.317s
8. Hugo Valente Campos SEAT 1m50.692s +1.596s
9. Rob Huff Munnich SEAT 1m50.745s +1.649s
10. Alex MacDowall Bamboo Chevrolet 1m50.753s +1.657s
11. Darryl O'Young ROAL BMW 1m50.885s +1.789s
12. Pepe Oriola Tuenti Chevrolet 1m50.925s +1.829s
13. Tom Coronel ROAL BMW 1m51.128s +2.032s
14. Rene Munnich Munnich SEAT 1m51.167s +2.071s
15. Stefano D'Aste PB BMW 1m51.243s +2.147s
16. Charles Ng Engstler BMW 1m51.268s +2.172s
17. Michel Nykjaer Nika Chevrolet 1m51.303s +2.207s
18. Fernando Monje Campos SEAT 1m51.391s +2.295s
19. Mehdi Bennani Proteam BMW 1m51.667s +2.571s
20. James Nash Bamboo Chevrolet 1m51.738s +2.642s
21. Marc Basseng Munnich SEAT 1m51.800s +2.704s
22. Franz Engstler Engstler BMW 1m51.981s +2.885s
23. Mikhail Kozlovskiy Lada 1m52.605s +3.509s
JAS Motorsport's Alessandro Mariani says he was "amazed" by Alexander Murdzevski Schedvin's suggestion that Volvo's structure compared to Honda was key to its 2017 World Touring Car Championship success
Volvo gave the World Touring Car Championship a major boost when it announced it would enter the series, but its ambitions don't stop there. JACK COZENS examines its programme, the S60 and what its arrival means for the WTCC
The 2012 World Touring Car champion says he never had the money to race cars. Yet 2015 is his 11th season in the WTCC. He talks STUART CODLING through his journey from motorsport fan to paid professional - for little more than £100,000