Saturday's killing was the second attack involving foreigners in Acapulco in less than three weeks. On Feb. 4, a band of masked gunmen invaded a beachfront home and raped six visiting Spanish women.
Police say the body of the Belgian was found at a supermarket parking lot in a Mercedes-Benz car with Mexico City license plates. Officials haven't released the victim's name but say he was 64.
The killing happened not far from the Princess Hotel, where federal police are guarding the tennis tournament.
Violence and crime have worsened in Acapulco in recent years, with drug gangs getting most of the blame.