Fletcher won the 1963 Kent Championships on grass at Beckenham, defeating Owen Davidson in the semifinal and Martin Mulligan in the final.

Fletcher won the 1966 British Hard Court Championships at Bournemouth on clay, defeating Tom Okker in the final.

His greatest success as a tennis player came in 1963, when he became the only man to win a calendar year Grand Slam in mixed doubles, partnering fellow Australian Margaret Court.

[1] Ken was a larrikin by nature, and many of his exploits feature in Hugh Lunn's books, especially Over the Top with Jim and Head Over Heels.

In later years, he was instrumental in gaining significant funding for medical research in Australia, through his association with Chuck Feeney.