Federer will turn 40 in August
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".
The 20-time Grand Slam champion is competing at this week's Qatar Open.
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.
Federer lost in straight sets to Djokovic at last year's Australian Open
"I know I need to go back to training after here again, so from this standpoint it's just about building up to being stronger, better, fitter, faster and all that stuff," he added.
"I hope then by Wimbledon I'm going to be 100%, and that's when the season starts for me."
Federer had initial arthroscopic surgery on his right knee last February.
A second operation was required in June as his knee would often swell up after walks and bike rides with his family.
"I wanted to go out on my terms, but for my life, I wanted to do this rehab anyhow. I want to go skiing with my children and my friends, and go and play basketball and football," he said when asked about his motivation to return at the age of 39.
"Retirement was never really on the cards. I think it's really a conversation more, let's say, if the knee keeps bothering me for months and months to come.
"I just feel like the story's not over yet. I enjoy playing tennis, I enjoy being on the road.
"Probably one of the other reasons to come back is that I would like to get that high again of playing against the best players, playing at the biggest tournaments, winning them hopefully, and being in the conversation."
During Federer's absence, Rafael Nadal has matched his mark of 20 Grand Slam titles by winning the French Open, and Novak Djokovic has moved onto 18 with victory at the Australian Open.
"My concern is more my own game, my own health, over the record," he said.
"I think for them maybe this is bigger than for me at this very moment because for me it was very important to equal the record of Pete [Sampras], and potentially break it," he said.
"For them, maybe I'm the measuring stick - like Pete was for me."
In Sunday's tournament finals, Russian Andrey Rublev, who beat Andy Murray and Stefanos Tsitsipas en route to a fourth successive ATP 500 final, won the Rotterdam Open with a 7-6 (7-4) 6-4 victory against qualifier Marton Fucsovics of Hungary.
The 23-year-old world number eight extended his winning run to 20 matches, winning in an hour and 50 minutes.
Meanwhile in Lyon, 18-year-old Danish player Clara Tauson won her first WTA title with a 6-4 6-1 victory over fellow qualifier Viktorija Golubic of Switzerland.
In just her third Tour-level main draw appearance, Tauson did not drop a set throughout the event.
-- Courtesy of BBC Sport