Borthwick, 24, was paired up with a new partner in Ana Veselinovic of Montenegro but the duo fitted together well in Redding, California.
The unseeded duo took down two ranked teams on their way to the final.
They began their run with a 3-6, 6-4, (10-1) upset victory over the No. 4 seeded team of Natela Dzalamidze and Angelina Gabueva and went on to beat Sandra Samir and Karman Kaur Thandi, 7-6 (8), 6-2, to reach the final.
There they faced the number three seeds, fellow Brit Harriet Dart and her American partner Maria Sanchez.
Again Borthwick and Veselinovic were to the fore as they won 6-3, 6-4 to lift the $25,000 ITF doubles title.
Borthwick spent five years at Florida State University completing a Master’s degree in 2016.
Since graduating she spent last year as assistant women’s coach at Columbia University in New York.
She is now assistant women’s coach at Wake Forest University in North Carolina, which has allowed her the flexibility to coach and play doubles.
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