British tennis star Andy Murray signs with Under Armour

Murray would become the brand's best-known tennis player. Sloane Stephens and Robby Ginepri already have Under Armour deals.

Murray, 27, is the 2012 US Open Champion and 2013 Wimbledon Champion. His five-year deal with Adidas, reportedly worth $3 million a year, is set to expire at the end of the year.

Under Armour "seems to be following Nike's formula for trying to get the big-name athletes," said sports marketing expert Bob Dorfman, executive creative director of Baker Street Advertising in San Francisco.

"Tom Brady, Murray – they are getting some big names," Dorfman said. "Obviously Nike has got the deepest pockets. But I think Under Armour has done a good job with the coolness factor. It's probably very difficult but I think they've done a really good job of cutting into Nike a little bit."

British media has been speculating for months about Murray's plans after the Adidas deal expires.

He was photographed recently wearing Under Armour gear during a training stint. The photo was widely reproduced on websites.

No formal announcement about the move has yet been made.

John Tobias, a Murray representative who is president of Lagardere Unlimited Tennis, declined comment on Tuesday as did Under Armour officials.

Under Armour overtook Adidas earlier this year in the United States in combined sales of apparel and footwear, taking the the spot as the No. 2 such brand behind Nike.

Athletes with Under Armour endorsements include swimmer Michael Phelps, football players Brady and Anquan Boldin, baseball players Matt Wieters and Bryce Harper, ballet dancer Misty Copeland and alpine skier Lindsey Vonn. It also sponsors the Premier League soccer team Tottenham Hotspur.

Last summer, Under Armour made a bid to sign basketball superstar Kevin Durant, a longtime Nike endorser. Under Armour. which became known for athletic performance apparel, is a relatively new player in footwear. Durant signed a new deal with Nike.

But just days later Under Armour inked a deal with Brazilian supermodel Gisele Bundchen to anchor its "I will what I want" campaign targeting women.

Under Armour scored one of its biggest sponsorship coups in January when it inked a deal with the University of Notre Dame, which, like Murray, also had been supplied by Adidas.