1921 U.S. National Championships (tennis)

The 1921 U.S. National Championships (now known as the US Open) was a tennis tournament that took place on the outdoor grass courts at the Germantown Cricket Club in Philadelphia, United States. The women's tournament was held from August 15 until August 20 while the men's tournament ran from September 9 until September 19. It was the 41st staging of the U.S. National Championships and the second Grand Slam tennis event of the year. Although Mallory won the women's singles event for the sixth time in seven years, her victory was overshadowed by her second-round win over Suzanne Lenglen, who was making her only ever competitive appearance in the United States. Lenglen retired from the match after losing the first set, the only competitive loss after World War I of her career.