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, admits that ten years ago she would never have expected to still be playing now.
Speaking ahead of the WTA Championships in Istanbul, Williams refused to rule out the improbable because she has already achieved what seemed to her impossible anyway.
That was the pulmonary embolism that 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 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."
As for the more immediate future, Williams thinks she can take her game to another level in the next year.
"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, she said.
"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, although her nearest rival, Belarussian Victoria Azarenka, has beaten her twice this year.
Williams starts Tuesday with a round robin match against Angelique Kerber, the leading German, and has to qualify from a group that 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.
