KEY BISCAYNE, Fla. (AP) -- Three-time champion Venus Williams withdrew from the Sony Open with a lower back injury shortly before her third-round match Saturday 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.''
Williams said she hopes the injury 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.''
Stephens, who is ranked a career-best 16th and upset Serena Williams at the Australian Open, could face the younger Williams sister again in the semifinals.
Venus 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.
Williams was playing at Key Biscayne for the 14th time. She won the title in 1998, 1999 and 2001.
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 moved ahead of No. 23 John Isner this week.
Andy Murray, the 2009 champion, needed only 56 minutes to beat Bernard Tomic 6-3, 6-1. Murray is seeded No. 2, and Tomic fell to 2-15 against top-10 opponents.
No. 6-seeded Jo-Wilfried Tsonga beat Viktor Troicki 7-6 (6), 6-3. No. 8 Richard Gasquet defeated qualifier Olivier Rochus 7-5, 6-2.
No. 18 Philipp Kohlschreiber lost to David Goffin 7-6 (5), 4-6, 6-2.
In women's play, defending champion Agnieszka Radwanska, seeded fourth, lost serve seven times and needed nearly 2 1/2 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.
No. 5 Li Na reached the fourth round by beating No. 25 Varvara Lepchenko of the United States 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.''