JOHANNESBURG (Reuters) - South Africa's Supreme Court of Appeal on Thursday dismissed an application by disgraced former grand slam doubles champion Bob Hewitt to challenge his conviction for rape and sexual assault.

Hewitt was sentenced to six years in prison in May last year after being found guilty of two counts of rape and a charge of sexual assault of minors.

Hewitt's lawyers had argued that the Australian-born Hewitt, 76, suffered from ill-health and also pointed to the length of time between when the offences were committed and the trial. The allegations first surfaced in 2011.

The Supreme Court said Hewitt's sentence was appropriate.

"In my view, the sentences fit the criminal and the crime and fairly balance the competing interests," the Supreme Court's deputy president Judge Mandisa Maya said in court papers.

It was not immediately clear when Hewitt would start serving his sentence.

Hewitt was a doubles and mixed doubles champion at all four grand slam events, winning seven Wimbledon titles. He was expelled earlier this year from the International Tennis Hall of Fame following his conviction.

(Reporting by Tanisha Heiberg; Editing by James Macharia)