Sharapova tumbled several times on the grass. With Sharapova trailing 3-2 in the second set, her right leg gave way behind the baseline and she did the splits.
Sharapova took a medical timeout and complained to the umpire about the grass conditions. She said later she believed she strained a hip muscle.
"I don't think I've ever fallen three times in a match before in my career, so that was a little strange," Sharapova said. "But that's certainly not an excuse.
"I think today I've seen a lot of players fall and take a few hits and a few injuries. So I think that's just part of the game, part of what we have to deal with."
Azarenka, the two-time Australian Open champion, pulled out after hurting her right knee in her opening-round win against Maria Joao Koehler. She withdrew minutes before her second-round match against Flavia Pennetta.
Azarenka reached the semifinals at Wimbledon the last two years and had been seeded to face Williams in the final.
It's only the second time in the Open era that a women's player seeded in the top 2 has conceded a match by walkover at any Grand Slam. The last time it happened was in 1974 at the French Open when second-seeded Nancy Richey pulled out before a match.
In her match against Koehler, Azarenka did the splits near the baseline, then crumpled to the grass, clutching her right knee and sobbing. She recovered after a medical timeout to win.
Azarenka said on Wednesday that medical tests showed she had a bone bruise rather than a tear but was unable to recover in time.
"We tried to do everything we could, but it was a very significant fall," she said. "To recover in two days after that seemed impossible."
Murray's victory came not long after his potential quarterfinal opponent, Tsonga, retired with a left knee injury while trailing Ernests Gulbis 3-6, 6-3, 6-3.
The Frenchman said he had been experiencing a problem with his knee tendon for a few days and it flared up during the match.
"I know when it's going worse and worse like this that it's not really good for me to play on because I know I will do more damage," he said.
Darcis withdrew a few hours before his scheduled match against Lukasz Kubot of Poland. He said he hurt his right shoulder while diving for a shot in the first set of his win against Nadal.
"After the match, a few hours after, I start to feel so much pain, I couldn't sleep that night," he said. "I saw the physio, the doctor, yesterday. They did a good job. It's a little bit better today. But no chance I can play."
Darcis had become an overnight sensation after beating the eight-time French Open champion.
"I was playing maybe the best tennis in my life here," Darcis said. "Not to go on the court today, it's maybe the biggest disappointing thing I have to do."
The tournament also lost 2002 champion Hewitt, who was ousted by 189th-ranked German qualifier Dustin Brown, 6-4, 6-4, 6-7 (3), 6-2.