The 12-lap race got off to a wild start, Dries Vanthoor firing his Audi into the wall at Mandarin as he tried to get a run on the pole-sitting Mercedes.
The Audi driver's race was effectively done on the spot, Maro Engel lucky to avoid a similar fate as he brushed the same wall while squabbling over fourth with Edoardo Mortara.
With Vanthoor out of the picture, Marciello spent the first lap with the two Rowe Porsches, Bamber leading Laurens Vanthoor, breathing down his neck. But a reprieve quickly arrived when the Safety Car was called out while the younger Vanthoor’s stricken Audi was cleared from Lisboa Bend.
The race went green again three laps later, but only briefly. As the field filed into the Lisboa Bend braking area Engel couldn't pull up his Mercedes, clipping Kelvin van der Linde's Audi before clattering into the back of an unsuspecting Mortara.
The clean-up lasted until lap 8, setting up a four-lap battle between Marciello and Bamber. But while the Kiwi was able to shadow the Team GruppeM Racing Mercedes there was no way past, and Bamber was forced to settle for second with Vanthoor in third.
"There was no point taking too much of a risk today, tomorrow is the main race," said Marciello. "It's a good starting position, now we just need to finish the job. To be in front after the start will be a big part of the job for tomorrow. We need to have a clean start and keep the lead into Lisboa, and then it will be in my control.”
Bamber added that the it was a "cat-and-mouse" game between Marciello's Mercedes and his Porsche, the two cars making their speed in different parts of the lap.
"We got a good start, and then it was a bit interesting in Turn 2 with the Audi going off, we had to avoid that," said Bamber. "We were strong in different places. The Porsche was good at the beginning of the lap, but a bit weak at the end of the mountain section. So it was a bit cat-and-mouse; we'd catch up and then lose a bit. It was a bit neutral."
Defending GT World Cup winner Augusto Farfus finished a fine fourth, coming all the way from 12th on the grid. The BMW driver was shuffled up the order as Vanthoor, Mortara, Engel and van der Linde had their issues, ending up fifth for the second restart. The Brazilian then executed a textbook move on Christopher Haase into Lisboa as the race went green.
Haase finished fifth ahead of Alexandre Imperatori, with Charles Weerts seventh, Kevin Estre eighth, Joel Eriksson ninth and Adderly Fong 10th. Van der Linde was classified 13th, repairs after the Engel contact putting him seven laps down.
Engel, meanwhile, will start tomorrow's main race from pitlane as a penalty for causing the crash with van der Linde and Mortara.