Tennis superstar Serena Williams has left her future in singles open as she prepares to make her return to court at Wimbledon next week. “I don’t know,” she told reporters on Saturday. “I can only tell you that I’m here. Who knows where I’ll pop up next. You just got to be ready.” The 40-year-old Williams, who has won 23 grand slam titles, has not competed individually since retiring from Wimbledon with an injury last year. She tuned up with Ons Jabeur in doubles last week and despite the risk of being rusty, said she had “high goals” for Wimbledon, even if she declined to go into details. Wil…