Three-time champion Venus Williams withdrew from the Sony Open with a lower back injury shortly before her third-round match against fellow American Sloane Stephens on Saturday in Key Biscayne, Fla.
Williams, seeded 19th, was extended to three sets in the second round on Thursday against Kimiko Date-Krumm. She said her back began to bother her on 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, South Carolina, from April 1.
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 on 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.
She'll play defending champion Agnieszka Radwanska on Monday, and could face a rematch against Serena Williams in the semifinals.
With evening traffic on the island in a snarl, five-time champion Serena rode a bicycle to her third-round match and beat Ayumi Morita 6-3, 6-3. Williams borrowed the bike at her hotel, fearful she might be late if she took a car.
"It was probably one of my best memories I think ever, riding a bike to a match," the No. 1-ranked Williams said. "That's pretty cool."
On a sunny, humid afternoon, second-seeded Andy Murray needed only 56 minutes to beat Bernard Tomic of Australia 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 lacklustre performance.
Jo-Wilfried Tsonga beat Viktor Troicki on hard-court for the first time by 7-6 (6), 6-3.
The No. 6-seeded Frenchman evened his record at 3-3 with Troicki, the Serb who'd won their three previous matches on hard-courts.
Troicki had set point at 5-4 and again in the tiebreaker, but Tsonga staved off both and broke Troicki twice in the second set.
Another Frenchman, No. 8 Richard Gasquet, defeated Belgian qualifier Olivier Rochus 7-5, 6-2.
Sam Querrey won in his first match as the top-ranked American man, 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).
No. 18 Philipp Kohlschreiber of Germany lost to David Goffin of Belgium 7-6 (5), 4-6, 6-2.
In women's play, defending champion 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 beat No. 25 Varvara Lepchenko of the United States 6-2, 6-4. Seeded players had their opening matches after receiving first-round byes.