The Portuguese driver hit the front from the outset, with his opening effort of 53.703s too good for the rest of the field.
With six minutes to run however he shaved an extra few hundredths off that time to end on a 53.683s, leaving him almost two tenths of a second clear at the top.
Izawa, a winner in Super GT this year, followed a similar pattern, with his opening effort enough to send him - and keep him - second overall.
Champion elect Yvan Muller was less than 0.1s down the road in third, while BMW man Tom Coronel prevailed in an extremely tight fight for fourth.
Norbert Michelisz, Pepe Oriola, James Thompson, Stefano D'Aste, Gabriele Tarquini and Rob Huff rounded out the top 10, with all six men finishing within four tenths of Coronel's time.
While there were no stoppages - in contrast to Friday's heavily interrupted test there were a multitude of incidents and moments throughout.
Muller, Huff and Michelisz were among several men to run off-track, while Masaki Kano drafted in to replace the ill Franz Engstler spun exiting Turn 8 and Marc Basseng slewed wildly into the gravel at Turn 2.
The latter ended the session down in 22nd, although still within 1.5s of pacesetter Monteiro.
Aside from Izawa, Hiroki Yoshimoto was best of the numerous newcomers, ending 11th fastest in the sole Nika Chevrolet.
Pos Driver Team/Car Time Gap 1. Tiago Monteiro Honda 53.683s 2. Takuya Izawa Honda 53.876s +0.193s 3. Yvan Muller RML Chevrolet 53.941s +0.258s 4. Tom Coronel ROAL BMW 54.093s +0.410s 5. Norbert Michelisz Zengo Honda 54.114s +0.431s 6. Pepe Oriola Tuenti Chevrolet 54.179s +0.496s 7. James Thompson Lada 54.268s +0.585s 8. Stefano D'Aste PB BMW 54.319s +0.636s 9. Gabriele Tarquini Honda 54.439s +0.756s 10. Rob Huff Munnich SEAT 54.493s +0.810s 11. Hiroki Yoshimoto Nika Chevrolet 54.497s +0.814s 12. Alex MacDowall Bamboo Chevrolet 54.554s +0.871s 13. Tom Chilton RML Chevrolet 54.561s +0.878s 14. Mehdi Bennani Proteam BMW 54.654s +0.971s 15. Fernando Monje Campos SEAT 54.677s +0.994s 16. James Nash Bamboo Chevrolet 54.682s +0.999s 17. Darryl O'Young ROAL BMW 54.842s +1.159s 18. Mikhail Kozlovskiy Lada 55.026s +1.343s 19. Masaki Kano Engstler BMW 55.034s +1.351s 20. Tom Boardman Special Tuning SEAT 55.064s +1.381s 21. Charles Ng Engstler BMW 55.139s +1.456s 22. Marc Basseng Munnich SEAT 55.154s +1.471s 23. Yukinori Taniguchi Wiechers BMW 55.156s +1.473s 24. Rene Munnich Munnich SEAT 55.616s +1.933s 25. Jeronimo Badaraco Son Veng Chevrolet 55.699s +2.016s 26. Filipe de Souza China Dragon Chevrolet 55.982s +2.299s 27. Henry Ho Engstler BMW 56.002s +2.319s 28. Kin Veng Ng China Dragon Chevrolet 56.501s +2.818s 29. Ka Lok Mak RPM BMW 57.400s +3.717s
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