Paris - American Jack Sock said he was shocked after moving from "thinking about his holidays" during his opening Paris Masters win, to qualifying for the World Tour Finals in London after claiming the Bercy title.
The 16th seed beat qualifier Filip Krajinovic 5-7, 6-4, 6-1 in the final on Sunday.
He trailed 5-1 in a deciding set during his second-round match against Britain's Kyle Edmund on Wednesday, but fought back before going on to snatch the eighth and final spot for the year-ending event from Spaniard Pablo Carreno Busta with the biggest title of his career.
Sock will rise from 22nd to ninth in the world rankings on Monday and is the first American to reach the Tour Finals since Mardy Fish in 2011.
"Coming in this week I had no idea I could even make London," the 25-year-old told a press conference.
"It wasn't in my head. I just wanted to play some good tennis my last week and go start my off-season.
"Now to sit here and extend the season another two weeks, and to keep playing and achieve a lot of firsts today for me, is incredible.
"I was thinking about my holidays the next day (when a double-break down to Edmund). I tried to start shortening points and just going after my shots. They were landing and I was connecting a lot of them."
Sock will also become the US number one for the first time in his career after winning his third ATP title of the season.
His victory was the first in a Masters tournament by an American since former world number one Andy Roddick took the Miami crown in 2010.
Sock said he would have nothing to lose when he arrives in London to take on the likes of Rafael Nadal and Roger Federer.
"I obviously snuck into the eighth spot in London unexpectedly, so zero pressure on me to go there," he added.
"I'm going to swing big and play my game like I always do, and just, honestly, enjoy the moment."
Serb Krajinovic played some excellent tennis in the opening set, but a poor service game to kick off the second proved costly as Sock roared back.
"I'm disappointed a little bit. I lost energy, I lost a physical moment in the match. First game of the second set I shouldn't lose that easy," he said.
"Then he started to play better and better and I didn't serve well today."
Krajinovic, also 25, is still set to rise to 33 in the world rankings on Monday, though, having dropped out of the top 200 last year after having surgery to remove an extra bone in his right wrist.
"Pressure is part of the sport. Everybody has pressure in life. Of course, I am going to have more," Krajinovic, who won five second-tier Challenger titles this season, added.
"People will expect more from me right now. They see I can do it. But if I want to be better, I have to handle that. So I will work on that also.
"I had to play Challenges because I didn't have a ranking. But now I will practice with those (top) guys and play with those guys and I think I can be dangerous."