Both were unhurt, Wheelchair Tennis South Africa said, but had their cellphones stolen in the incident near a highway in Johannesburg at around 11.15 Friday night.
"I am glad my car, wallet and my life are still with me," Sithole said in the statement.
Sithole's car was also damaged after the armed men emerged from a vehicle in front of the brothers on a highway onramp and forced them out of their car, WTSA said.
"Although Lucas is shaken and traumatized, he was not physically injured or shot and is recovering well," the statement said.
Sithole, 27, lost both his legs and his right arm in a train accident in 1998. He took up tennis and won the 2013 U.S. Open title, becoming the first African player to win a wheelchair tennis Grand Slam event.
South Africa's crime problem has been raised as an issue at the murder trial of double-amputee runner Oscar Pistorius, who is a multiple Paralympic champion, Olympic athlete and South Africa's most famous disabled sportsman. Pistorius, who is also 27, claims he fatally shot his girlfriend by mistake last year thinking she was a dangerous intruder in his home.
Sithole competed at the 2012 Paralympics for South Africa alongside Pistorius.
"We are extremely grateful that Lucas and his brother were not physically injured during the hijacking and will make a speedy recovery from the trauma," Sithole's coach Holger Losch said.