Knapp began the day with her rain-delayed semi-final against number one seed Sara Errani and needed more than two and a half hours to prevail 7-6 (7/4) 4-6 7-5.
The Italian looked unstoppable as she went 6-4 5-4 up against Stosur and was serving for the match, but her opponent recovered to win the second set on a tie-break before cruising to a 3-6 7-6 (7/3) 6-2??victory to add to the title she won in Strasbourg in May.
Knapp told wtatour.com: “It wasn’t easy to play five hours today but I was really fighting. I just missed some chances and she used hers.”
Stosur sympathised with her opponent and said: “It’s not ideal to be playing a semi-final and final in one day, a two-and-a-half-hour match in the morning then another two and a half hours in the afternoon, but unfortunately that’s just what happens with the weather sometimes.
“And she did serve for the match, so she wasn’t doing too badly either!”