(CNN) -- Two-time champion Venus Williams is out of the U.S. Open following a battling 6-3 2-6 7-6 (7-5) second-round defeat to world No. 56 Zheng Jie.
Former world No. 1 Williams, currently ranked 60th, fought bravely against her Chinese opponent before eventually losing in three hours and two minutes.
The marathon encounter was the joint fifth-longest women's singles match in the tournament's history.
In recent years Williams has struggled with form and fitness and entered the New York tournament with a back injury. She was diagnosed with the auto-immune disorder Sjogren's Syndrome at Flushing Meadows two years ago.
"I think there's a lot of positives for me out of this tournament. I continue to serve better and recover [from] my back injury with each tournament," the 33-year-old told the event's official website.
"I'll just continue to step up and give myself chances. A lot of times I'm putting myself in a good position but not always capitalizing. So I'll get there. I just have to keep working at it."
Williams' compatriot Sloane Stephens is at the opposite end of her career and the 15th seed continued her rise to prominence with a commanding 6-1 6-1 defeat of Poland's Urszula Radwanska.
"I came out with a really good attitude and I think it showed," said the 20-year-old, who plays fellow American and 23rd seed Jamie Hampton in the second round.
"I think every point was the only thing on my mind. That's what happens when you really, really, really want to win."
Third seed Agnieszka Radwanska, Urszula's older sister, fared better than her sibling, beating 103-ranked Spaniard Maria-Teresa Torro-Flor 6-0 7-5.
In the men's draw defending champion Andy Murray made light work of his first-round match with Frenchman Michael Llodra.
The 2013 Wimbledon winner, seeded third, secured a 6-2 6-4 6-3 win to set up a tie with Argentina's Leandro Mayer.
"I felt comfortable before the match. I didn't feel any extra pressure, which was good," said Murray, who is defending a grand slam title for the first time.
"I was just ready to play... There were no of unknowns out there on the court. I've played many matches there ... I didn't feel loads of nerves before the match. I just wanted to get on."
American veteran James Blake bid farewell to his home grand slam with an agonizing five-set loss against Croatia's Ivo Karlovic.
The 33-year-old Blake announced on Monday this would be his last tournament, bringing the curtain down on a 14-year professional career.
Two past champions will meet in the second round after both Juan Martin del Potro and Lleyton Hewitt chalked up opening victories.
Sixth seed Del Potro saw off spirited opposition in the shape of Spain's Guillermo Garcia-Lopez, while former world No. 1 Hewitt beat U.S. wildcard Brian Baker.