July 23 (Reuters) - World number one Novak Djokovic has withdrawn from next month"s U.S. Open tune-up event in Toronto, Tennis Canada said on Friday. A four-time champion in Canada, Djokovic is at the Tokyo Olympics where he will try to keep alive his hopes of achieving a "Golden Slam" - victories at all four Grand Slam tournaments in a single year plus an Olympic singles gold. The Serbian has already won the first three majors of the season. "It would have been fantastic to have hosted Novak," Karl Hale, tournament director of the Aug. 9-15 National Bank Open, said in a news release. "However, following a great run to his record-equaling 20th Grand Slam title at Wimbledon and his participation at the Olympic Games in Tokyo, we understand his decision. We wish him all the very best for the rest of the season." Because of Djokovic"s withdrawal from the hardcourt event, one of a handful of tune-ups ahead of the Aug. 30-Sept. 12 U.S. Open, Russian Daniil Medvedev will be the Canadian tournament"s top seed. Tennis Canada also said Austrian Dominic Thiem, Belgian David Goffin and Swiss Stan Wawrinka, who are each dealing with injuries, have also withdrawn.