Serena Williams, who wants to win more Grand Slam titles than any other player, has not ruled out being able to play until she is 40 years old.
Williams, who has won 17 Grand Slams so far and needs another five to beat Steffi Graf's open era record, admitted that ten years ago she would never have expected to be here still playing now.
When asked where she expected to be in another ten years, the 32-year-old replied: "Who knows? I might be here. I have learned from ten years ago. I can never say."
Speaking ahead of the WTA Championships which start here on Tuesday, Williams refused to rule out the improbable because she has already achieved what seemed to her impossible anyway.
That was the pulmonary embolism which endangered her life three years ago and forced her to spend almost a year away from the WTA tour.
"It doesn't matter what happens to me in the present I never give up," she said. "If you have an opportunity to survive, you survive.
"I had overwhelming emotions when I returned to number one again," she said of the triumph which happened in February this year. "I never expected that again.
"I didn't think I would play again. I just wanted to try to survive. It put a lot of character in me and even more mental toughness.
"It made me realise I have overcome a lot of things. I had a lot of help with that. I think all of that really, really made me feel I can do it."
As for the more immediate future, Williams said: "I never thought I would have one of my best years this year, and now there is an opportunity to do even better.
"I am interested for next year to take my game to a new level, which I think I can do.
"Even if I got to 23 Grand Slams, it would be very hard, you know, with so many wonderful players, especially now. There are so many great new players coming up. Everyone is so young, everyone is so hungry, everyone wants to be the next number one.
"So, you know, it's going to be even tough for me to get to 18. Obviously that's my goal, but I take it one match at a time."
Williams is firm favourite to retain the WTA Championships title and win it for a fourth time. However her nearest rival, Victoria Azarenka, the Belarussian, has beaten her twice this year and ran her close in the US Open final last month.
"I think it's great that, I don't know, I'm always considered a favourite in a tournament," the American said. "I don't think about it. I don't like it, but I don't hate it. You know, it's better to be considered a favourite.
"I also do well when I'm considered the underdog. I never consider myself a favourite because every opponent I play has a chance to do really well."
Williams starts Tuesday with a round robin match against Angelique Kerber, the leading German, and has to qualify from a group which also contains Sara Errani, the Italian number one, and Petra Kvitova, the former Wimbledon champion from the Czech republic, if she is to reach Saturday's semi-finals.
She and Azarenka are seeded to meet in the final the day after that.