Welcome back to Ancient Wisdom, our Sunday series in which writers over 70 tell us how they are aging gracefully. Last week, the novelist and playwright Anthony Giardina, 74, wrote about why he’s not ready to hang it up. This week, tennis great Chris Evert, 70, writes about surviving cancer—and how it changed her.

You have to be selfish when you play an individual sport like tennis.

I started playing junior tournaments when I was eight years old, turned professional in 1972 when I was 18, and retired in 1989 at the age of 34. During that time—a span of almost three decades—I was always preparing for the next match, the next tournament. By the time I stopped playing, I was tired of being so focused on myself. I know there are athletes who have a hard time moving on after their career is over, but that wasn’t me. For most people, retirement is something that happens in their 60s, not their 30s. But I’d sacrificed so much for tennis that I was ready to embrace everything else that life had to offer. Instead of viewing retirement as a chapter ending, I saw it as a whole world opening up.

I wanted to have children, and I did: three boys in the space of five years. Motherhood gave me so much joy. Sometimes I would be approached about making a comeback or playing World TeamTennis, but I always said no. I wanted to spend every minute with my kids. Thanks to my tennis earnings, I didn’t have to worry about money, so I had the luxury of choice. I could wake up every day and do what I wanted. I was married to Andy Mill, an Olympic downhill skier, for 20 years. We had homes in Boca Raton, where I grew up, and Aspen, where he grew up. We had a good life.

I became involved with the Women’s Tennis Association, which has always maintained close ties to former players, and which for me felt like a kind of sisterhood. And I kept track of the players I had competed against. After all, they were my friends too: Martina Navratilova, Monica Seles, Pam Shriver. And Billie Jean King, who paved the way for all of us in women’s tennis, was a friend too. Fifteen years ago, when I was 55, I became a commentator. Newly single, with my kids out of the house, I needed something that would keep me engaged.

My brother John and I started the Evert Tennis Academy in Boca Raton shortly after I retired. When my kids were small, I wasn’t on the premises much, but as they grew up and I had more free time, I started going every day. I became a mentor to the young players, especially the young women, listening to them and advising them. Whenever you have big experiences in life, that creates wisdom, and I had tennis wisdom. I’ve been in so many demanding situations, under so much pressure, that I could talk to them about dealing with and handling stress. I found that kind of mentoring very fulfilling. I found my whole life fulfilling.

Then I got cancer...