Under the merciless midday sun, Sloane Stephens was in the unenviable position of battling a cold in the midst of the heat.

She bent over to catch her breath, yelled out profanities in frustration, and rolled her eyes at her own uncharacteristic mistakes. But for all the obstacles standing in the way of her United States Open title defense, the most challenging proved to be the indomitable woman on the other side of the net.

Anastasija Sevastova, the No. 19 seed from Latvia who was seeking her first U.S. Open semifinal berth, withstood the oppressive conditions to defeat the third-seeded Stephens, 6-2, 6-3, in a blazing-hot Arthur Ashe Stadium on Tuesday.

Stephens, who captivated the tennis world a year ago with her surprise championship here and a glowing post-victory celebration, acknowledged fighting a bad sinus infection that was compounded by the heat. But she conceded that the better player had won.