Marciello, who will leave the marque after this weekend ahead of an anticipated move to BMW, left it until the very end of the 30-minute session to set the fastest time, a 2m14.542s, in his Team Landgraf Mercedes-AMG GT3.
That was more than a second underneath the existing lap record, set by the Italian/Swiss driver in qualifying for the previous edition of the GT World Cup in 2019.
"The car felt amazing," commented Marciello. "The track was very fast, and already from practice, I could feel the track was really quick compared to previous years.
"This year is pretty cold, so the track is faster, but the record doesn't really matter."
Edoardo Mortara had looked on course for pole in the closing minutes as he became the first driver to clear the 2m15s barrier, setting a 2m14.758s with a little over five minutes left on the clock.
But the Team Absolute Audi driver had to be content with second, 0.216s off the pace.
"Congrats to him [Marciello]," said seven-time Macau winner Mortara. "I was pretty happy with my lap, I pretty much nailed it when I had peak grip of the tyres. There was nothing more I believe I could have done, but we missed a little bit.
Source: Autosport