"I don't feel like I need to prove anything, but I want to challenge myself and I want to be strong again," she said on the WTA Insider Podcast. "This is my marathon. This is where I'm saying, 'OK, let's try this.'"
Clijsters, 36, said she will be on the Women's Tennis Association (WTA) Tour in 2020, playing in the competitive circuit, but in a more flexible schedule to account for her family.
"If I feel like it's interfering with what's going on with the kids, then I'm not playing. Then I'll wait until it fits," she said.
This will be Clijsters' second return to the sport. In 2007, two years after winning her first Grand Slam at the U.S. Open, she announced her retirement at age 23, citing injuries and her desire to have children.
But two years after that, in 2009, after giving birth to her daughter, Jada, Clijsters, who is from Belgium, made her first comeback -- and it was one for the history books.
In that initial comeback, Clijsters won the U.S. Open in 2009 and 2010 and the Australian Open in 2011. In doing so, Clijsters became just the third woman to win a Grand Slam after becoming a mother, joining greats Margaret Court and Evonne Goolagong (While Serena Williams has been in several Grand Slam finals since becoming a mother, she has not yet won one).
She announced retirement again in 2012 after the U.S. Open, but after continuing to watch competition, she told the WTA Insider Podcast, she's been itching to get back on the court herself.
"Whenever I'm at a Grand Slam ... if somebody asked me, 'Hey, do you want to hit some balls?' I'm the first one to be like, 'I'll hit; I'll be the hitting partner for your practice today.' I still love playing tennis," she said.
Now a mother of three -- to Jada, 11, Jack, 5, and Blake, 2 --Clijsters has been focusing on getting her fitness back, she said. As her kids are now largely in school, she has more time, and she discussed the decision with them as well as with her husband, team and the WTA.
Clijsters will also be benefiting from her past accomplishments. As a previous champion, she has the right under WTA rules to request wild-card admission into major tournaments, so she doesn't have to necessarily play all of the competitions needed to qualify for the Grand Slams.
"That's the great thing about the system that I didn't know existed. The fact that when you've been No.1, you've won a Grand Slam, and you want to come back then you can ask for wild cards," she said.
Other notable returns to the sport include Martina Navratilova, who retired in 1994 but returned in 2000. She retired again in 2006, at the age of 49, right after winning mixed doubles at the U.S. Open.
Monica Seles returned to competition in 1995, two years after being stabbed by a man on-court. She went on to win the 1996 Australian Open.
And then there was Bjorn Borg, whose return from an early retirement after eight years was not quite as successful.
While the 2019 tennis season is coming to a close, Clijsters will be seeing where she's at as 2020 starts -- and as the Australian Open kicks off -- to decide if she feels fitness-ready enough to jump into competition immediately.
For now, Clijsters is enjoying the feeling of pushing herself again.
"I'm surprised how, at times when I'm going through a rough practice, how easy it is for me to stick with it and to fight through it and to push through it," she said on the podcast. "It's a very satisfying feeling to have that kind of challenge again."