New US number one John Isner, who took back-to-back titles the past two weeks in Newport and Atlanta, withdrew from the ATP Citi Open on Monday with a right knee injury.
The 32-year-old standout, a three-time runner-up at the Washington hardcourt event, won the 11th and 12th titles of his career over the past two weeks, downing Australian Matthew Ebden on Newport grass and compatriot Ryan Harrison in two Atlanta tie-breakers in the finals.
But the toll prompted Isner to withdraw from Washington, where he was seeded ninth and owned a first-round bye, as Masters Series events in Montreal and Cincinnati loom before the US Open starts August 28.
The twin triumphs were enough to boost Isner over Jack Sock in the rankings to top the American list, Isner moving to 18th this week with Sock staying at 19th.
Isner's departure leaves only nine of the world's top 20 players at the US Open tuneup event in the US capital.
© 2017 AFP