NEW YORK — Preparing for what she knew would be her last professional tennis tournament, Kim Clijsters devoted her effort and energy to the U.S. Open.

NEW YORK — Preparing for what she knew would be her last professional tennis tournament, Kim Clijsters devoted her effort and energy to the U.S. Open.

Family time with her 4-year-old daughter Jada needed to wait a tad longer.

That wait is over.

Clijsters lost 7-6 (4), 7-6 (5) to 18-year-old Laura Robson of Britain in the second round of the U.S. Open, and will head into retirement after she finishes playing doubles at Flushing Meadows.

She walked away from the sport once before, in May 2007, then returned after a hiatus of two-plus years. Now 29, the Belgian insisted this season that she means it this time, and decided the U.S. Open — and its hard courts that she conquered on the way to three championships — would be her final tournament.

"It's the place that has inspired me so much to do well and to do great things. It's hard to explain sometimes why," Clijsters said in an on-court interview, her face flushed and her eyes welling with tears.

"This completely feels like the perfect place to retire," Clijsters told the spectators at Arthur Ashe Stadium, many of whom rose to shower her with a standing ovation. "I just wish it wasn't today."

Clijsters was the only woman of real significance who lost on Day 3, when the winners included No. 1 Victoria Azarenka, four-time major champion Maria Sharapova, defending champion Sam Stosur, 2011 Wimbledon champion Petra Kvitova and 2011 French Open champion Li Na.

Joining Robson with a surprise victory was American wild-card entry Mallory Burdette, the NCAA runner-up who reached the third round in her Grand Slam debut by eliminating 69th-ranked Lucie Hradecka 6-2, 6-4.

Earlier on Ashe, the highest-ranked American man, John Isner, let out a big exhale of relief while waving to the crowd after getting past an argumentative Xavier Malisse 6-3, 7-6 (5), 5-7, 7-6 (9) in the first round.