The Dutchman is deputising for Alex Albon and is set to start the race from eighth on the grid after jumping into the FW44 for FP3.
He qualified 13th but has moved up five places thanks to penalties for other drivers.
De Vries, who drive in FP1 for Aston Martin in Monza, was relaxing in the Paddock Club when Mercedes strategy chief James Vowles contacted him and told him that there was a question mark over Alex Albon’s fitness.
He immediately went to Williams and sat in on a pre-session briefing before it was officially confirmed that Albon had appendicitis and would not be able to take part in the rest of the weekend.
"I was having a coffee in the Paddock Club for an appearance, and it was quiet early in the morning," he said when asked by Autosport how he learned of his chance.
"I was watching the F3 podium with a cappuccino. And then James Vowles gave me a ring, and I had to come down quickly to Mercedes, and everyone was pretty happy. And I kind of tried to manage expectations a little bit, saying let's wait.
"So then I went down to Williams and unfortunately Alex was ill, but hopefully he will be back soon and recover quickly. And then during the FP3 briefing, it kind of became clear that it was my duty to be in the car for FP3 and the rest of the weekend.
"This all happened only one and a half hours prior to FP3, so there was not a lot of time to quickly get my head into it."
De Vries, who serves as reserve for Mercedes, Williams and Aston Martin, admitted that he never expected the job to lead to an opportunity to race.
"You spend so much time travelling and it never quite happens, so you almost lose the thought of actually expecting it could happen, and this was obviously not even before Friday, but very late on a Saturday, so it was very last minute.
"But maybe it was even good, because then you don't get a chance to think, and you've just got to get on with it."
De Vries made it out of Q1 in 15th place, despite losing his fastest time to track limits. He then earned 13th in Q2 after aborting his second run when he went off at the second chicane after inadvertently hitting the brake balance switch.
"You only have two runs in FP3, because you have two sets of tyres, and you won't do any long runs. So there is no time for getting slowly into it, you have to just kind of push yourself, to force it and learn quickly to get ready for qualifying. And I think we did a good job on that.
"I do feel there was more in it, especially in the last run of Q2 I hit a brake balance switch which moved the brake balance a per cent rearwards, and that's why I looked at my rears going into Turn 4, and lost my lap.
"But ultimately to get through to Q2 and be in the mix I think was a decent job on such a short notice."
De Vries admitted he enjoyed the fact that penalties for others had moved him into the top 10.
"Yeah, that does make me smile a little bit, even though I'm hard on myself and think I should have done a better job. That is great.
"But I also want to make sure we do our homework tonight and get ready and hopefully we can build our race, have a good start, have a good first few laps, and then get into it, find a bit of a rhythm.
"I obviously have no clue I didn't do any long running whatsoever. And then yeah, maybe dream of a point."
