"It was a good start at the beginning of the match. In the second set, it was tough physically and mentally and I couldn't make it. It was almost finished, but not!" Doi said after the match.
The 24-year-old Japanesewoman led Barthel by a set and a break before the big-hitting German broke back and eventually leveled the match in a tie-break.
Falling behind early in the first game of the third, Doi recovered from 0-40 down to hold and went on a roll from there.
"The key point was that first game. It was 0-40 but I tried to focus on every single point and built confidence from there."
Doi broke serve three more times in the final set to clinch her first WTA title in her first final, an encouraging way to end a solid season.
"It was an incredible moment and I'm just happy!"
For Barthel, the day wasn't over; despite losing in the singles final, she earned redemption in doubles; she and countrywoman Laura Siegemund upset top seeds Anabel Medina Garrigues and Arantxa Parra Santonja, 6-2, 7-6(2) to nonetheless end the week with a title.
"I think Misaki played a really good match today and a good tournament. I tried everything and it was my last singles match of the year, but in the end, she was just a little bit fresher and fitter. But I was happy to be in the final, and I think it was a great week."