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 day'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.
"I'm a little bummed, but I guess a win is a win," Stephens said. "On to the next round."
She'll play defending champion Agnieszka Radwanska on Monday, 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."
Williams 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.
The 32-year-old Williams was playing at Key Biscayne for the 14th time. She won the title in 1998, 1999 and 2001.
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."
Williams' 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.
On a sunny, humid afternoon, Sam Querrey won in his first match as the top-ranked American, rallying past Lukasz Kubot 4-6, 6-3, 6-3. The 20th-ranked Querrey this week moved ahead of No. 23 John Isner, who hit 25 aces and beat Ivan Dodig, 6-4, 5-7, 7-6 (5).
Isner has won his past five winner-take-all third-set tiebreakers.
"I really tend to play my best in a tiebreaker, for whatever reason," he said. "I wish I could sometimes play a little better outside of that."
Second-seeded Andy Murray needed only 56 minutes to beat Bernard Tomic 6-3, 6-1. The absence of Roger Federer and Rafael Nadal helps Murray's chances in the tournament, which he won in 2009.
"When you know they aren't going to be here, then it obviously changes things a little bit," Murray said. "For tennis it's way better if all of the top players are here. I think we have been very lucky the last few years especially that a lot of the top players have fulfilled their commitments to all of the big events. So it's going to happen from time to time, and it's unfortunate for this event that it happened here."
Tomic, who fell to 2-15 against top-10 opponents, drew jeers from the stadium crowd during his lackluster performance.
Radwanska, seeded fourth, lost serve seven times and needed nearly 2½ hours to beat Magdalena Rybarikova 7-6 (5), 2-6, 6-3. No. 9 Caroline Wozniacki lost to wild card Garbine Muguruza of Spain, 6-2, 6-4.
Querrey lost his first service game but held the rest of the way and hit 12 aces against Kubot. The 25-year-old Querrey said being the top-ranked American has provided only a modest boost in confidence.
"Some days more than others," he said. "I never thought about it out there today. Haven't really thought about it much. Felt like another day out there to me. Nothing new."
Querrey won despite a lack of rest. He's staying near downtown Miami, site of a weekend electronic music festival.
"I'm not sleeping well because of this Ultra Festival going on," he said. "I could hear the bass, and strobe lights were shining in my hotel, so it's hard to sleep."
Isner said the festival hasn't bothered him.
"I can sleep through anything," he said. "I can sleep through a train wreck. Do not hear it, which is good."