Leander Paes, who was admitted to a Florida hospital on Sunday with a suspected lesion in his brain, has reacted "very positively" to treatment.
"He is feeling much better," his elder sister Jacqueline told rediff.com on Friday morning.
The Indian tennis ace is much more active, can exercise and do yoga, his sister said.
Doctors at the Orlando Regional Medical Center in Orlando, Florida, have conducted a battery of tests on Leander. They will repeat the tests, Jacqueline added, and the results should be available in the next 24 to 48 hours.
Leander checked himself into the hospital near his Orlando home after suffering from severe headaches for three days last week.
While not yet proven in Leander's situation, a doctor was quoted as saying one common infection in India causing such a brain lesion is cysticercus granuloma, caused by the larval form of a tapeworm.
"Leander does not eat red meat, but the doctors say the infection may have come from eating salads or vegetables," his sister said.
The 1996 Olympic bronze medallist, who turned 30 on June 17, no longer has headaches; his vision too is normal.
Leander's father, 1972 Olympic bronze medallist Dr Vece Paes, left for Orlando on Thursday and is expected to arrive there on Friday night.