Roger Federer says he never seriously considered retirement in his 14 months away from tennis, and is returning as he feels "the story's not over yet".
It will be his first tour appearance since losing to Novak Djokovic in last year's Australian Open semi-finals.
"I know it's on the rare side for almost a 40-year-old to come back after a year being out. What's important is I'm injury and pain-free," he said.
"The pain is completely under control."
The 39-year-old has a bye into the second round in Doha, where he will play either Britain's Dan Evans or Jeremy Chardy of France.