MGM’s tennis-based romance “Challengers” opened this weekend atop the North American box office, taking in an estimated $15 million behind positive reviews and the powerful net game of actor/singer Zendaya, industry watcher Exhibitor Relations reported Sunday.”This is a strong opening for a romance and sports drama,” said analyst David A. Gross, crediting Zendaya’s star power and the directorial talents of Luca Guadagnino (he of “Call Me By Your Name”). “This is the right material with the right director, featuring the right star at the right time,” Gross said.Zendaya plays a tennis…