KEY BISCAYNE, Fla. -- A big match beckoned for Venus Williams on a court where she first hoisted the championship trophy 15 years ago.
Alas, on Saturday she wasn't up to any heavy lifting.
The three-time Key Biscayne champion withdrew from the Sony Open because of a lower-back injury shortly before her third-round match against fellow American Sloane Stephens.
Williams, seeded 19th, was extended to three sets in the second round Thursday against Kimiko Date-Krumm. She said her back began to bother her Friday, and she decided after warming up that she couldn't play.
"It's really disappointing," she said. "But I have faced disappointments in my life and my career. It's not the first; probably not the last."
The cancellation cost the tournament the session's most appealing match, pitting a rising star against a seven-time Grand Slam champion. The 20-year-old Stephens upset Serena Williams at the Australian Open and is ranked a career-best 16th.
Stephens will next play defending champion Agnieszka Radwanska and could face a rematch against Serena Williams in the semifinals.
Venus Williams said she hopes the back ailment won't prevent her from playing in her next scheduled tournament at Charleston, S.C., beginning April 1.
"That's always a concern, but I have dealt with injuries before in my whole career," she said. "I feel like also I know how to hopefully recover quickly from them."
Her injury was the second in two days to sideline a former Key Biscayne champion. Two-time winner Victoria Azarenka withdrew before her opening match Friday because of a right ankle injury.
The 32-year-old Williams was playing at Key Biscayne for the 14th time. She won the title in 1998, 1999 and 2001.
But she hasn't beaten a top-10 opponent since August, and has played in just three tournaments this year. She lost in the third round at the Australian Open to Maria Sharapova, and was beaten by No. 109-ranked Olga Puchkova in the semifinals of the Brazil Tennis Cup last month.
Stephens said it was difficult to imagine herself playing for so many years.
"Oh, man. That's crazy," she said. "I don't know. I just turned 20, so I don't know. I mean, I hope. I mean, if I can I want to, but that's still really a long ways away for me."