KEY BISCAYNE, Fla. — A big match beckoned for Venus Williams on a court where she first hoisted the championship trophy 15 years ago.

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. Five-time champion Serena won her third-round match, overcoming a 3-love deficit in the first set to beat Ayumi Morita 6-3, 6-3.

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.

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.

Radwanska, seeded fourth, lost serve seven times and needed nearly 21/2; 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.