The 125th-ranked German, who had never been beyond the quarterfinals of an ATP event before, had to play two qualifying rounds to reach the main draw and became only the third qualifier this season to win an ATP tournament.
"This is unbelievable, I can't describe my feelings now,'' Petzschner said. "I will enter the top 100, which makes a dream come true.''
The 24-year-old Petzschner played his opening qualifying match less than three hours after arriving on a flight from Tokyo. He went on to beat top-seeded Stanislas Wawrinka, former No. 1 Carlos Moya and 2004 champion Feliciano Lopez on his way to the title.
"This has been an incredible week for me, everything went perfect,'' Petzschner said.
The eighth-seeded Monfils has lost five of his six career finals.
"I am disappointed but I had a good week anyway,'' said the 28th-ranked Frenchman, who had beaten Petzschner in straight sets in their only previous match last month. "Philipp was the better player today. He just was more aggressive so well done to him.''
Monfils will enter the ATP top 20 for his highest ranking ever.
Petzschner closed out the victory with a volley on his first match point.
"This win gives me a lot of confidence, I now see what I have been working for all year,'' Petzschner said. "My goal next season will be to get into the top 50.''
Petzschner and Alexander Peya of Austria lost 6-1, 7-5 to Max Mirnyi of Belarus and Andy Ram of Israel in the doubles final.
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