The American was a break down at 1-3 in the first set when she withdrew.
“I’m sorry I couldn’t do it today, I tried,” said the 23-time Grand Slam champion. “It’s been a tough year, but we’ll keep going.”
“Serena, you made me cry. I know how it is to pull out of tournaments, it’s not easy,” she said.
“This wasn’t the way I expected to win. You are truly a champion. I’ve watched you win so many times, you are truly a champion on and off the court.
“I’m speechless right now. This has been a dream come true.”
Williams was seeking to win her first title since returning to competition in 2018 after the birth of her daughter.
She reached the Wimbledon final last month – losing to Simona Halep – but struggled with a knee injury earlier in the year.