Antonelli will start the race in Monaco at the front of the field after the Mercedes man went just 0.043 seconds faster than Max Verstappen in qualifying, with Lewis Hamilton also close behind.
"It was a really good lap, I managed to put everything together," he said afterwards.
"It was close. I didn't expect Max and Red Bull to be there. They did an amazing job."
It was clear that Verstappen would be in with a real chance of claiming pole when he topped Q2, but Antonelli just focused on himself through the final run, feeling that it was too intense to keep an eye on his rival's times.
"I didn't know Max's time," the championship leader said.
"I felt like I did a good lap. Everything flowed well. I didn't know where I stood... It's so intense here that all you think about is driving as fast as possible."
The Italian admitted that prevailing took a lot out of him, such was the stress and intensity.
"It was very intense. I feel a bit shaken. It was a lot. When it comes down to the end, it's never easy."
With the narrow circuit making overtaking near-impossible, whoever starts the Monaco Grand Prix at the front usually finishes as the leader too. However, Antonelli has often lost places at the start of races this season, and Verstappen is renowned for being willing to take huge risks in his efforts to pass rivals.
"I know who's behind me. They're going to push and put pressure on me, but we'll try to get a good start," said Antonelli.