Tennis star Andy Murray just dealt a major blow to Under Armour

(AP)

Tennis star Andy Murray has dealt a major blow to Under Armour's fledgling tennis business.

Murray is under a four-year sponsorship deal with the Baltimore-based retailer.

Under the contract, he's supposed to wear Under Armour gear on the court.

But at the US Open, which began August 31, Murray has been wearing Adidas on his feet, USA Today reports.

Adidas highlighted its contract-breaking victory over Under Armour on Tuesday in a jeering tweet.

"You can choose armour, or you can choose a weapon," the brand tweeted, with a photo of Murray wearing his Adidas shoes.

Murray was previously sponsored by Adidas.

Under his new contract with Under Armour, which was signed in January, Murray was allowed to continue wearing the Adidas Barricade shoe until Under Armour designed a new shoe for him according to his "exact specifications," USA Today reports.

(Under Armour didn't make a tennis shoe at the time that he signed the contract.)

Over the course of several months, Under Armour developed a shoe for him called the Clutchfit Pursuit.

The process took longer than expected because Under Armour's shoe designers had to fly all over the world to meet Murray and show him shoe samples, according to Sports Illustrated.

Murray finally debuted the Under Armour Clutchfit Pursuit in June at Queen's Club. But last month, he was back to wearing his Adidas on the court.

Here's Murray wearing the Under Armour shoes in June:

(AP)

Murray's decision to return to his Adidas shoes could be a bad omen for Under Armour, which launched its tennis business this year.

It won't have much of an impact on sales for now, however, since Under Armour doesn't currently sell a tennis shoe to the mass market.

The company only made one pair of Clutchfit Pursuit shoes specifically for Murray, and it hasn't revealed plans for a wide release yet.

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