After the Olympics, Talbot left to become a coach in Canada, forcing Cooper to return to Brisbane to train with Harry Gallagher. In 1973, he was the Australian Champion in the 200 m, 400 m and 800 m freestyle, and the 100 m and 200 m backstroke. However, he continued to switch coaches throughout the year, also training with Allan Wood and Tony Fraser. He went on to compete at the 1973 World Aquatics Championships in Belgrade, where media interest surrounded his 400 m and 1500 m freestyle races against DeMont. In the 400 m, both broke the world record and broke the four-minute barrier for the first time, with DeMont finishing ahead of Cooper. Cooper also finished behind DeMont in the 1500 m, but the race was won by Australian Stephen Holland, who broke his own world record. Cooper also won a silver medal as part of the 4 × 200 m freestyle relay. Cooper was then awarded a swimming scholarship to Miami University, but was unable to enrol as he had not finished his secondary education in Australia.