Playing in his first Australian Open at the age of 27, Baker had just won the first set in his second-round match against fellow American Sam Querrey when his body gave out on him again.
He crumpled to the court after hitting a shot long and shouted in pain, grasping his right knee. After limping awkwardly and then hopping to his chair, he sat down, ripped off his headband and shook his head in frustration.
Baker retired several minutes later and was pushed off the court in a wheelchair. As the crowd applauded, a woman shouted from the stands, "Don't give up, Brian."
"He said he kind of just felt his knee almost buckle and kind of heard like a pop or a snap," No. 20 seed Querrey said after the match. "He didn't know if it was bones or a tear, but he couldn't straighten it, couldn't walk."
Tournament organizers said later that Baker had a torn meniscus. He is expected to be out about four months.
While the injury could have been worse, the setback is no doubt a heartbreaking one for a player who has undergone five major surgeries—but never had a knee problem until Wednesday.
"He's the last person that deserves anything like that," said Querrey, who has become friends with Baker as they've both come back from injuries over the past year. "He does everything right, treats his body great, just trying to come back, and then something like that happens, it's just so unlucky."
Baker's injury elicited messages of support from other players who have followed—and admired—his comeback.
"It's a shame," said fellow American Tim Smyczek after his second-round loss to Spaniard David Ferrer. "He's such a good player. He's got so much talent and he's got great tennis IQ. He's just had the worst luck."
Baker was once the No. 2-ranked junior in the world and a boys French Open finalist in 2003. He recorded his first top-10 victory at the U.S. Open in 2005 when he upset then-No. 9 Gaston Gaudio in the first round. He lost his next match against Xavier Malisse—and then didn't play another ATP-level match for seven years.
Any of the surgeries Baker underwent during that time could have ended his career: left hip (2005), sports hernia (2006), right elbow reconstructive surgery (February 2008), left hip again (April 2008), right hip (June 2008).
Baker took a job as an assistant tennis coach at Belmont University in his native Nashville, Tenn. But he never ruled out a comeback on the pro tour.
Baker got his chance at a small ATP tournament in Nice last May. Ranked No. 216, he qualified for the main draw and then made it all the way to the final, beating 13th-ranked Gael Monfils along the way. Weeks later, he qualified for the main draw at Wimbledon and reached the fourth round, pushing his ranking up to 76th.
Baker started the new year full of promise, closing in on the top 50 in the rankings. He beat Russia's Alex Bogomolov Jr. in the first round at Melbourne Park—his first-ever five-set victory—setting up the second-round encounter with Querrey.
"He has been so unfortunate in the last six years, seven years," Querrey said. "So, you know, I think if he can heal quickly or if it takes a year, I think he can get right back where he is right now. He's talented; he's good enough. No matter what happens, I still think he can come back to where he is now."