Complex sentence: Tonelli took up swimming due to his asthma, and quickly came to prominence, winning Australian selection for the 1973 World Championships, coming sixth in the 200 m backstroke at the age of 16.
Split into simpler sentences: Tonelli took up swimming due to his asthma, and quickly came to prominence. Selected to represent Australia at the 1973 World Championships, he came sixth in the 200 m backstroke at the age of 16.