Nadia Petrova wrenched the Pan Pacific Open title from holder Agnieszka Radwanska as the Russian shocked the world number three 6-0, 1-6, 6-3 in a topsy-turvy final in Tokyo.
"I feel amazing. This is the biggest tournament I ever won and this is a great accomplishment," said a jubilant Petrova, ranked 18th in the world.
"At the beginning of the tournament I didn't expect to be here because I had a back injury, so it's delightful to be here right now."
The victory caps a highly satisfying week for the 30-year-old after she stormed back from one set and 1-4 down to beat sixth seed Sara Errani and went on to beat eighth seed Samantha Stosur in the last two rounds.
For Petrova, whose only previous win over Radwanska came in Tokyo in 2008, it was a second title of the season following her victory at 's-Hertogenbosch, and 12th overall.
She was handed a winner's cheque of $385,000 and will move up to 14th in the world.
The 23-year-old Radwanska, who became Poland's first Grand Slam finalist of the Open Era at Wimbledon this season, had to be satisfied with the runner-up prize of $192,000.
She said: "Her biggest weapon is her serve, for sure, but to be honest, her return was not bad today. I really had to serve very good to win my game, so her serve and return were really working for her today.
"I was really trying to get the title again this year, but unfortunately she was playing too good."
Petrova fired a sizzling service return winner on a break point to take an early 1-0 lead before whitewashing the first set by finishing off with an ace on her second set point.
A struggling Radwanska saved four break points to keep her service in the first game of the second set and pulled off her first break when Petrova hit a forehand into the net to move up 2-0.
Petrova lapsed into making errors and took only one point each in the following three games before keeping the sixth game, but Radwanska hit three aces in a row and a service winner to comfortably even it at one set all.
Neither had a break chance in the final set until the score went to 4-3 for Petrova, who took a break point in the eighth game thanks to Radwanska's two double-faults.
Petrova broke the game with a backhand winner to go up 5-3 and calmly served out for the match, hitting a forehand volley winner on her second match point.