The dreadlocked Brown, who switched nationality from Jamaica in 2010, was in tears after beating the Australian.
"I cried like a little girl," said Brown, who has played mainly on the lower-tier challenger circuit in 2013 and had never won a match at Wimbledon until this year.
Ninth-seeded Wozniacki slumped to a 6-2, 6-2 loss to 196th-ranked Czech qualifier Petra Cetkovska, continuing her disappointing run since finishing 2011 as No. 1.
The Dane said she slipped during the match and her foot was bothering her.
"I didn't feel 100 percent out there after I slipped," she said. "I overstretched the foot a bit."
In another women's match, Canadian teenager Eugenie Bouchard picked up the biggest victory of her career, beating 12th-seeded Ana Ivanovic, the 2008 French Open champion, 6-3, 6-3 to reach the third round.
Isner, the 18th-seeded American, retired during his second-round match against Adrian Mannarino of France after only two games.
Isner took a medical time out during the second game and a trainer taped the knee. But he was clearly hobbled and decided to quit after serving the first point in the third game.
The retirement came three years after Isner beat Nicolas Mahut 70-68 in the fifth set at Wimbledon in the longest match in tennis history. Now, Isner is out after one of the shortest matches at Wimbledon.
Isner said he felt something in his knee when he landed on his left leg after hitting a serve on the third point of the match.
"I always serve and land on my left leg, like I have done 20 million times playing this game, and this is the first time I just felt this sharp pain," he said. "It didn't pop. It just grabbed like really badly, and I knew I was in serious trouble then. I knew at that point it was not likely I was going to be able to play."
Isner said it appeared to be a tendon injury, not a major tear, and he does not expect to need surgery.
"I just can't bend my knee," he said. "I can walk as long as I keep it straight."