Delfs is especially noteworthy for winning men's singles at the first IBF World Championships held in Malmö, Sweden in 1977. He dominated that same 1976/1977 season by winning nearly all other noteworthy tournaments, including the All-England Championships. Delfs won three consecutive European men's singles titles in 1976, 1978, and 1980. He played on all four DanishThomas Cup (men's international) teams between 1972 and 1982, two of which (1973, 1979) reached the championship round before losing to Indonesia. Tall, with an elegant style and powerful backhand, Delfs was a highly impressive player at his best, but typically had difficulty in the hot, humid conditions he encountered in the Far East.