Story highlights Four "ball dogs" handle retrieving duties during an exhibition at the Brazil Open

Frida, Mel, Isabelle and Costela were rescued from slums and vacant lots around Sao Paulo

"We want to show that abandoned dogs can be adopted and trained," say trainer Andrea Beckert

(CNN) Apparently, you can teach an old dog new tricks.

To prove the point, four shelter dogs rescued from slums and abandoned lots around Sao Paulo were trained as "ball dogs" for an exhibition tennis match this week at the Brazil Open.

The dogs playfully ran after the balls that went out of play and reluctantly returned them to the players and dog trainers during the match this week.

"We want to show that abandoned dogs can be adopted and trained," said Andrea Beckert, a trainer from the Association of Animal Wellbeing. "After all, it's not easy to get a dog to only pick up the lost balls, and then to give them up!"

The four dogs were Frida, Mel, Isabelle and Costela.

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