MELBOURNE, Australia – After being sidelined by injuries for nearly six years, Brian Baker was finally back on the court with a second chance at a professional career.

Playing in his first Australian Open at the age of 27, Baker had just won the first set 7-6 (2) in his second-round match against fellow American Sam Querrey when his body gave out on him again.

At 1-1 in the second set Tuesday (PST), Baker crumpled to the court after hitting a shot long and shouted in pain, grasping his right knee. After limping awkwardly and 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," the 20th-seeded 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."

While the severity of the knee injury wasn't immediately known, the setback was no doubt heartbreaking for a player who has undergone five major surgeries but had never had a knee problem.

"He's the last person that deserves anything like that," said Querrey, who has become friends with Baker as both have 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 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.

"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."

Meanwhile, fourth-seeded David Ferrer needed five match points before clinching a 6-0, 7-5, 4-6, 6-3 win over American Tim Smyczek, who got into the main draw as a lucky loser from qualifying. Fifth-seeded Tomas Berdych beat Guillaume Rufin of France 6-2, 6-2, 6-4.

On the women's side, Agnieszka Radwanska extended the best winning streak of the year to 11 matches as she reached the third round with a 6-3, 6-3 victory over Romania's Irina-Camelia Begu.

The fourth-ranked Radwanska won leadup tournaments this month in Auckland and Sydney.

"I can play even better," said Radwanska, who lost the Wimbledon final to Serena Williams last year. "I didn't really expect I could win that many matches in a row, and hopefully I can keep going."

Her last two trips to the Australian Open have ended in quarterfinal losses to the eventual champion: Kim Clijsters in 2011 and Victoria Azarenka last year.

With the top-ranked Azarenka and Williams on the other half of the draw, Radwanska's main obstacles to reaching the final are sixth-seeded Li Na, who beat Olga Govortsova of Belarus 6-2, 7-5, and potential semifinal rivals Maria Sharapova and Angelique Kerber.

The fifth-ranked Kerber advanced with a 6-3, 6-1 win over Lucie Hradecka of the Czech Republic, and the second-ranked Sharapova was to face Japan's Misaki Doi.

Williams hurt her ankle in her opening victory, causing speculation that her bid for a third consecutive major title could be in jeopardy. She canceled a practice session Tuesday.

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