Having missed the Jeddah race, Sainz spent two weeks focusing on his recovery before returning to the cockpit for Friday practice in Albert Park.
Although he was fit to drive, his preparations were compromised by skipping training and not running in the Ferrari simulator.
Despite that he had the edge on team-mate Charles Leclerc in qualifying and having passed the ailing Max Verstappen in the opening laps, he went on to lead a one-two for the Italian team.
"This winter we agreed together to push until the last lap of the season," he said when asked by Motorsport.com to sum up Sainz's performance.
"And he did a fantastic job. He was there in Bahrain, pushing the team also, and for sure Jeddah was a tough weekend, a tough weekend for Carlos, a tough weekend for the team.
"But the recovery is mega. You have to keep in mind that two weeks ago he was in the hospital. I think even Friday he was not 100 per cent sure to be able to drive.
"And after a couple of laps, he was on the pace, and this was part of the success – because you can't give up one lap in free practice if you want to perform.
"For sure, it's amazing. If you have a look on where he's coming from nobody expected a result like this today."
Vasseur downplayed the fact that Sainz had an edge on team-mate Leclerc.
"We are speaking about plus or minus half a tenth," he said. "And classification is one thing, the pace is another one.
"I think during the race the issue is that we had to cover [Oscar] Piastri at the beginning, that was not ideal in terms of race time, for sure.
"We didn't want to give up the position on track, and we put him in a tough situation. But on the last stint, he knew more about the degradation of the hard, and he was able to be very consistent and it was a good one."
