By Mark Gleeson

CAPE TOWN (Reuters) - Chad le Clos's much-anticipated re-match with superstar Michael Phelps in Rio de Janeiro's Olympic pool is clouded by a family health scare that hangs heavily over the 24-year-old South African.

Le Clos announced earlier this month that both his parents were being treated for cancer, casting an emotive shadow over his preparations for Brazil where the 200 meters butterfly duel is arguably the must-see event at the Olympic Aquatics Stadium in Barra da Tijuca.

Le Clos narrowly pipped the American in the race at London, a dramatic victory that catapulted the youngster and his colorful father Bert into the spotlight.

Bert le Clos became almost as famous as his son with his demonstrative reaction to his success and for memorable TV interviews in London that revealed a close family bond.

Bert had surgery for prostate cancer in June while Chad le Clos's mother Geraldine recently had a double mastectomy and is undergoing chemotherapy.

“It has not been an easy time but I am training hard for Rio," said Le Clos, who is hoping for gold in the 100 and 200m butterfly and also competing in the individual medley.

"More than anything else I want (my parents) to win their battles. I also hope they will be in Rio."

Le Clos, who also won silver in the 100m butterfly at London, has had vacillating form through the Olympic cycle.

He heads to Rio ranked fourth in the world in the 200 butterfly ahead of sixth-placed Phelps and 12th in the 100m behind the second-ranked American.

But Le Clos is a proven performer on big stages, having won the 100m at last year's world championships in Kazan and taken silver in the 200. Phelps was absent for the meet.

Phelps was Le Clos's idol as the South African's career took off but there is now something of an edge to their rivalry.

Phelps has made clear his desire for revenge and Le Clos has been accused of some trash talking since London.

But with his parents ill, it is expected to be a more somber atmosphere around the Le Clos camp this time.

"Getting into the pool, I feel I'm unshackling myself from the worry and stress. Swimming definitely takes my mind off things," Le Clos told the New York Times.

(Editing by Ian Ransom)