Sloane Stephens last night announced on her Instagram and Twitter feeds that she is endorsing Nike. Stephens’ contract with Under Armour expired after last year’s U.S. Open, which she won for her first Grand Slam championship. A source said UA tried to secure Stephens, as did Adidas and Uniqlo. Nike counts most of the top tennis players in its stable of endorsers, including Serena Williams , Maria Sharapova , Roger Federer and Rafael Nadal . Stephens is repped by TLA Worldwide. While Stephens has a new apparel and sneaker deal, current WTA Tour No. 1 Simona Halep remains without one after she could not come to terms with Adidas .