Kim, the co-owner and president of the NFL's Buffalo Bills and NHL's Buffalo Sabres, suffered a cardiac arrest in June and needed CPR from her other daughter Kelly before paramedics arrived and restored her heartbeat.
Pegula received the news after returning home to Florida from the French Open, where she lost to eventual champion Iga Swiatek but rose to number eight in the world.
"Suddenly I went from, 'Let's celebrate top 10 in the world' to, 'Do I need to start thinking about my career after tennis a lot sooner than I thought?'" Pegula wrote in an essay in The Players' Tribune.
"I'm 28 and I take pride in being able to handle every situation thrown at me, but this was a lot."
Pegula said she wanted to share her mother's story after Bills safety Damar Hamlin suffered a cardiac arrest during an NFL game last month.
Pegula went on to play Wimbledon and the U.S. Open last year to reach a career-high ranking of number three.
"I still wanted to play Wimbledon if I knew my mom was O.K.," Pegula wrote. "My dad didn't want me to play, but I knew she would be upset if I skipped because of her.
"I had to deal with a lot of speculation and questions surrounding her health, even shutting down rumours that she had died," added Pegula, who lost to Victoria Azarenka in the quarter-finals of this year's Australian Open.
"It wasn't necessarily the most fun Wimbledon experience I remember. I had a few good wins, and I was proud I was able to go out and compete considering the situation."
