When a superskinny girl in layered sweats recently strolled down a Miami street, onlookers may have thought they had just witnessed a serious celebrity sighting.

Oversized sunglasses, a floppy hat, a ginormous bag — it must be Mary-Kate Olsen! But a double take revealed that the anorexic-looking blonde was a different celeb altogether — former tennis ace Anna Kournikova.

Once professional tennis’ most voluptuous and marketable star, Anna Kournikova, 25, has lost a lot weight since retiring from the tour due to a back injury in 2003.

Kournikova says she eats “less than half” the calories she took in when she was playing, performs hour-long cardio workouts at least four times a week and goes to Miami trainer Shihan Moti Horenstein for lessons in an intense Israeli martial-arts form called Krav Maga, which mixes aerobic training with self-defense techniques.

But Anna may be taking her training too far. Dr. Edward Jackowski, author of Escape Your Shape: How to Work Out Smarter, Not Harder, says 5′6″ Anna’s body fat is too low and estimates that her weight has plummeted from a healthy 135 pounds to about 115 pounds.

“For a woman of her height and body type, she should weigh no less than 130 pounds,” he explains. Dr. Jackowski, who does not treat Anna, says it’s more likely that the court princess’ weight loss is due to a combination of overexercising and not eating enough.

“Many ice skaters and gymnasts also suffer from overtraining syndrome,” he says. “Anna should lay off the frequency of her exercise regimen and start eating more healthy foods to put back on some good weight.” [via]