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Rafael Nadal.
Uniqlo star Novak Djokovic beat Nike's Roger Federer in the men's final, while Adidas-sponsored Flavia Pennetta took the women's final.
Nike star Maria Sharapova dropped out of the competition, while Rafael Nadal failed to advance past the third round.
The losses are costly considering the brands pay athletes exhorbitant sums to wear their gear. It's estimated Nike pays Nadal $10 million per year.
Under Armour's British tennis star, Andy Murray, wore Adidas shoes and lost.
According to Investor's Business Daily, Under Armour's tennis shoes are still under development and were not ready for game time. The brand has tennis shoes designed for clay and grass, but not the hard court surfaces of the US Open.
Under Armour will provide a sales update on September 16. Analysts expect a whopping growth of 20-25%.
Nike has ruled the sportswear sector for years.
But Under Armour has slowly been eating into Nike's market and is now the second most-popular athletic apparel brand in America. Adidas lags behind at number three.
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Under Armour's Andy Murray had to wear Adidas shoes on the court.
The brand's secret to success isn't to imitate Nike's strategies. Instead, the brand focuses on what makes it different to drive sales.
Under Armour marketing executive Adrienne Lofton told Fast Company that the brand considers itself an "underdog brand."
"We work with athletes who most people wouldn't or didn't draft in the first round, or who they wouldn't traditionally give a prima ballerina title to. We pick that athlete with a chip on their shoulder and their desire to win because it aligns with our own attitude," she told Fast Company.