Johnson is breathing independently after using a ventilator for several weeks, most recently only at night, his doctor, Vivek Deshmukh, said Tuesday.
The South Dakota Democrat, who underwent emergency surgery on Dec. 13, remains in intensive care at George Washington University Hospital, said his spokeswoman, Julianne Fisher.
"The senator continues to make progress," Fisher said. "The next step would be rehabilitation, and we hope that would happen within the week."
A tracheotomy tube remains in his neck to guard his airway, and the senator still has not spoken, Fisher said.
The senator is expected to leave intensive care but remain in the hospital when he begins the rehabilitation process. His office has said his recovery is expected to take several months.
Johnson was diagnosed with the arteriovenous malformation, a condition, often present from birth, that causes arteries and veins to grow abnormally large, become tangled and sometimes burst. He was rushed to the hospital after becoming disoriented on a phone call with reporters, and he underwent surgery hours later.
His long-term prognosis is unclear. Doctors have said he is steadily improving and has been responsive to his family and physicians, following commands, squeezing his wife's hand and understanding speech. He needed the ventilator to assist with his breathing after fluid developed in his lungs as a consequence of his initial hemorrhage, his doctors said.
In cases like Johnson's, doctors often depend on the patient's ability to answer questions to assess any cognitive damage caused by the hemorrhage.
The senator's sudden illness raised questions about the Democrats' one-vote majority in the upcoming Senate session. South Dakota's Republican governor, Mike Rounds, would appoint a replacement if Johnson's seat were vacated by his death or resignation.
A Republican appointee would create a 50-50 tie and could effectively allow the GOP to retain Senate control because of Vice President Dick Cheney's tie-breaking vote.
Johnson's wife, Barbara, called his change in condition "wonderful news."
"Tim has come a long way and we have a long way yet to go, but we celebrate each victory," Barbara Johnson said. "For me, it is like watching a miracle unfold each day."