Federer, who spent more weeks at No. 1 than anyone, already had reached his lowest spot in a decade by dropping to No. 5 after Wimbledon — and he fell two more places Monday.
Barring a withdrawal, Federer will be seeded seventh at the U.S. Open, where he won five of his 17 Grand Slam titles.
Canada's Milos Raonic slipped out of the top 10, dropping one spot to No. 11. Raonic, fromThornhill, Ont., was beaten by John Isner of the United States in the third round of the Cincinnati Open.
Vasek Pospisil of Vernon, B.C., moved up one spot to No. 39.
In other changes, French Open champion Rafael Nadal — who won hard-court titles at Montreal and Cincinnati the last two weeks — rose to No. 2 from No. 3, swapping with Wimbledon champion Andy Murray. Novak Djokovic remained No. 1.
Isner, runner-up to Nadal at Cincinnati, returned to the top 20, jumping to 14th from 22nd.