The 22-year-old beat the second seed Novak Djokovic 7-5 6-4 after 97 minutes of intriguing tennis.
In the aftermath of his victory, Khachanov, long touted as one of the next stars of the ATP circuit, said his four set loss to Nadal in New York convinced him he had the ability to compete with the alpha males of the circuit. Nadal beat him 5-7, 7-5, 7-6, 7-6 after four hours and 22 minutes.
“I saw that my level was there and that I could play and compete on this level. It was a matter of just a few points,” said Khachanov. "I saw that if I continued to do the same things that I was doing it would turn around on my side. I was really believing in this. And actually that's what I've got.”
How it turned. Khachanov showed some verve and no little nerve to come back from 1-3 down in the opening set to win it 7-5 in 58 minutes.
After claiming the early break in the second set, he had three chances to lead 5-2 with a double break.
But Djokovic, attempting to win a 33 Masters crown, managed to hold on to his serve. Unpeturbed, Khachanov fired down a ninth ace on his way to a 5-3 lead.
Serving for the match at 5-4, Khachanov unveiled the full range of weapons. A foray to the net for a forehand volley brought him to within two points of the title. Match point came courtesy of a 200 kmh serve and a straightforward backhand winner.
He held his arms aloft in triumph when a Djokovic backhand landed out.
“It's one of my biggest titles so far, my biggest achievement. And in general, it's a breakthrough season," beamed Khachanov. "And with this title, it's a good way to end the year. And although I'm not crying, I'm really happy."
Djokovic, who will be officially installed as the world number one on 5 November, went into the final as the favourite. But though he refused to blame Saturday’s three hour semi-final victory over Roger Federer for any sloppiness, the Serb was not his usual pitiless self.
“Karen showed great quality today and he showed why we're going to see a lot of him in the future,” said Djokovic.
While Djokovic leaves Paris to prepare for the eight man end of season championships in London as top seed, Khachanov revealed his study schedule for the off-season.
“I don't mind this attention because you have to deal with the pressure I think at every stage. If you think about the top guys and look at what they are doing when they go on court;
For example, Novak went on the court with me and he was the favourite. They have to deal with this pressure. And in most of the situations and matches, they deal with it.
“It's work. It's a homework that I have to do. And with experience I think it will come.”