Serena Williams says she frequently wondered how she would keep going after returning to the practice courts following the birth of her first child.
The 23-time Grand Slam singles champion will return to the WTA Tour at Indian Wells in California this week.
In an interview with BBC Sport, the 36-year-old American said she is motivated by the thought of playing long enough for her six-month-old daughter to have memories of watching her.
"It's been hard," she said.
"There have been so many days, even still, when I'm like, 'how am I going to keep going?