The pair spent almost the entire session at the top of the timesheets, with only a brief salvo from the RML Chevrolet of Yvan Muller denying them a clean sweep.
It was Monteiro who began fastest, getting to within two thousandths of a second of Norbert Michelisz's 2012 pole time in the opening nine minutes.
Muller moved ahead at the one-third distance mark, but within one minute Tarquini had usurped him with a 2m12.229s that put him half a second clear of the field.
The time also held until the chequered flag, when Tarquini shaved another 0.015s off to cement his dominance of the weekend's opening session.
Monteiro followed him home, just under four tenths of a second slower.
Third went to the ROAL BMW of Tom Coronel, which - like all the E90s barring that of Franz Engstler - has been given a 20kg weight break for the weekend.
Muller and RML stablemate Tom Chilton were fourth and fifth respectively, albeit both at least half a second down the road from Tarquini.
Defending champion Rob Huff went sixth in his Munnich-run SEAT, narrowly edging the Bamboo Chevrolet of Alex MacDowall and Marrakech race winner Pepe Oriola's Tuenti SEAT.
A trio of BMWs rounded out the top 11, with Freddy Barth - who enjoyed one lurid moment at Turn 3 - heading Darryl O'Young and Mehdi Bennani.
Like BMW, Lada has also been given a 20kg weight break but it did not translate into immediate gains. James Thompson ended the session 18th fastest, four places ahead of Russian team-mate Mikhail Kozlovskiy.
Pos Driver Car Time Gap Laps 1. Gabriele Tarquini Honda 2m12.215s 10 2. Tiago Monteiro Honda 2m12.612s + 0.397s 11 3. Tom Coronel ROAL BMW 2m12.693s + 0.478s 11 4. Yvan Muller RML Chevrolet 2m12.757s + 0.542s 11 5. Tom Chilton RML Chevrolet 2m13.024s + 0.809s 11 6. Robert Huff Munnich SEAT 2m13.285s + 1.070s 9 7. Alex MacDowall Bamboo Chevrolet 2m13.412s + 1.197s 12 8. Pepe Oriola Tuenti SEAT 2m13.503s + 1.288s 10 9. Fredy Barth Wiechers BMW 2m13.754s + 1.539s 9 10. Darryl O'Young ROAL BMW 2m13.940s + 1.725s 13 11. Mehdi Bennani Proteam BMW 2m13.955s + 1.740s 11 12. Michel Nykjer NIKA Chevrolet 2m14.009s + 1.794s 11 13. Norbert Michelisz Zengo Honda 2m14.072s + 1.857s 8 14. James Nash Bamboo Chevrolet 2m14.076s + 1.861s 12 15. Charles Ng Engstler BMW 2m14.163s + 1.948s 8 16. Stefano D'Aste PB BMW 2m14.627s + 2.412s 13 17. Fernando Monje Campos SEAT 2m14.646s + 2.431s 9 18. James Thompson Lada 2m15.039s + 2.824s 8 19. Marc Basseng Munnich SEAT 2m15.637s + 3.422s 11 20. Tom Boardman Special Tuning SEAT 2m16.166s + 3.951s 10 21. Franz Engstler Engstler BMW 2m16.200s + 3.985s 6 22. M.Kozlovskiy Lada 2m16.290s + 4.075s 11 23. Rene Munnich Munnich SEAT no time 0
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