"It feels great," said the Frenchwoman, who did not drop a set throughout the tournament and lost only 24 games in her five matches.
"It's been over a year since I won a tournament. I still had a lot of good tennis left in me and that's what keeps me motivated," she added.
"It puts the word out that I've won a hardcourt title here in the U.S. Hopefully, I can keep it going."
After playing a singles semi-final and long doubles semi-final on Saturday evening, the unseeded Sugiyama was unable to muster the energy necessary to push Pierce in the 75-minute contest.
The top-ranked Japanese player, who took a medical timeout for a split toenail after the third game of the second set, only managed to hold serve once in the contest and was broken four times to love.
With Pierce on the verge of victory, Sugiyama staged a late revival when she fought off five match points before breaking her opponent to pull back to 5-2.
Sugiyama then held her own serve and battled gamely in the last game, with the two times grand slam champion needing three more match points to seal the contest.
Pierce erred on a forehand on the first of them and missed a simple overhead on the second that had both players doubling over with laughter.
"You couldn't have asked for an easier shot to put away the match. What can you do other than just laugh," Pierce said.
STUNNING WINNER
However, Pierce regained her composure to rip a stunning forehand winner and an ace to take the match.
"Sometimes my level is really high," "Pierce said.
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It was the first final between two 30-year-olds on the WTA Tour this year and the Japanese player was magnanimous in defeat.
"She just played so great, so aggressively every point and it was tough because she was all over every ball," Sugiyama said.
With the victory, French Open finalist Pierce will rise to number 11 when the WTA rankings are released on Monday.
Pierce has not been ranked in the top-10 since April 2001.
"When you are younger, winning titles is really exciting but now it means more because I know how much work I've had to put into it," said Pierce, who has suffered numerous serious injuries over her 16-year career.
After playing in Los Angeles next week, Pierce will take a two-week break before heading to the U.S. Open, where she has never advanced past the quarter-finals.
"I would definitely like to do better than that," Pierce added.
"Clay has always been my favourite surface, but this title gives me confidence going into the U.S. Open."