The Russian has never been exactly plump, but observers in America fear she's losing the curves which catapulted her to fame in the late '90s and developing a more athletic, slender frame.
The 26-year-old blonde, who last played professional tennis in 2003, ran four miles of the Nautica South Beach Triathlon in Miami on Sunday to help raise money for St Jude's Children's Hospital.
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However, unlike her fellow competitors, Anna sat out the swimming and biking part of the triathlon, but happily rejoined them later for the post-sporting cocktail party.
Following the run, Anna said: "It was really cool. I am a little out of breath, sorry. I was expecting nothing really because I was sick a week or two ago.
"I tried to stay focused and sucked it up and just went. The beginning was hard. The first mile felt like forever.
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Keep on running: Anna Kournikova competes in the four-mile race section of the Nautica South Beach Triathlon in Miami on Sunday
"My teammates are amazing athletes and incredible women, I give them a lot of respect."
At the triathlon after-party on a yacht moored at Miami Beach, Anna declared her six-year relationship with Latin pop star Enrique Iglesias was still going strong, but the pair had ruled out marriage.
She told People magazine: "I'm never getting married. Everything is good."
Anna's tennis physique: Anna, pictured at a charity match in 2004, was more muscley and curvy during her tennis days
