Starting from pole for the first time in his premier class career, Marini was overtaken at turn two by Maverick Vinales before eventual race winner Jorge Martin also came through a few laps later.
But despite that, Marini was strong throughout the race as he began making a late charge back towards Martin.
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With the win in his sights, Marini was preparing for a move on the final lap, before a mistake at turn 13 cost him that chance.
Marini did manage to claim a comfortable second place finish which left him ‘proud’ after competing with an injured collarbone.
“Proud of the whole day,” said the Italian. “Since the Misano test I am feeling great with the bike. I made a huge step forward but then I decided to break my collarbone in the worst part of the season.
“I complicated everything and this is a big pity because I’m feeling great. Today the bike was great and capable of winning but I was not in a good condition.
“From lap four it was really difficult in the left corners. Fortunately here we don’t have so many left-hand corners.
“We will see tomorrow because it will be a completely different race. My strategy will be completely different because today I knew I could stress my body to 100%, but tomorrow we will have to manage.”
With the grand prix set to include a further 14 laps, Marini admits his strategy will need to be different than in the sprint.
Marini said: “Now I’m not feeling so good. I would like to go to the swimming pool as soon as possible and do some physiotherapy to try and rest.
“Tomorrow will be really tough. The medium rear will help me a bit because when you are riding with the hard rear tyre your body stress is less, because there is very low grip.
“But with more laps it will be really tough so the strategy is to not stress too much my body because I want to arrive at Phillip Island in good condition.
Turn 13 mistake ‘a pity’
Teammate Marco Bezzecchi has grabbed many headlines so far this season, and rightfully so for his stunning performances.
But the sprint in Mandalika looked like Marini's chance to do so after grabbing pole and being a contender throughout the 13-lap race.
After seeing Martin struggle for rear grip, Marini closed in by over six tenths in two laps prior to his error.
Marini added: “I saw that Jorge was struggling with the tyres and wanted to make a bit of time, but then the entry to turn 13, I had very high temperature in the rear tyre because I was spinning a lot in the corner before.
“When I entered with a bit too much rear brake I lost the rear. I lost one second which is a pity because maybe I could try a move on the last lap.”