Rublev was the better man from the outset as he blasted his way to a 6-3 604 victory.
It was Rublev’s first-ever win over Swiss and just his second victory over a top ten player.
As the pair embraced at the net, they exchanged words and Rublev began to cry.
Speaking to the ATP after the match, Rublev revealed what Federer said to him and why it meant so much.
“It is impossible to explain. The way he finished the match was so amazing,” he told the ATP.
“I told him it’s a big honour and he said no worries.
“The way he did it was amazing. Now I feel that he is a true legend. That’s what I’m feeling inside.”
Rublev has struggled with injury in his short career but has enjoyed a strong year.
Federer has a formidable record in Cincinnati with seven titles but he was unable to keep up with Rublev, who showed very few nerves.
When asked how he stayed so calm, Rublev replied: “To be honest, I don’t know. I respect him a lot. I respect all the top players a lot.
“But today when I was going on [the] court I was going to win. I respect him a lot during the match. I was thinking I will not just go to enjoy.
“I will go to fight, to try to win, give my best and then we’ll see what happens. And that’s it.”