For a guy with a reputation for—how shall we put this?—emotional volatility, Andre Agassi can play things surprisingly cool. Take the conversation the world's currently top-ranked tennis player had a few weeks ago with his older sister Tami. "I was talking to him and asked him about Steffi," she recalls, referring to Agassi's girlfriend of nearly two years, retired German tennis ace Steffi Graf. "He told me she was pregnant."

On July 12 Agassi let the world in on the news, confirming rumors that he and Graf, 32, the former No. 1 women's player, were expecting their first child in mid-December. "This is a very exciting time for us," he said. "We are so happy to have been blessed with this gift."

According to Tami, Agassi, 31, is "excited about every aspect of becoming a father." But he and Graf— who divide their time between his Spanish-style spread in his hometown of Las Vegas and a $23 million compound they are renovating in Tiburon, on San Francisco Bay—have apparently chosen to stay in the dark about the baby's sex. "I know they'd like a boy," says Agassi's mother, Elizabeth. But "they have the results in an envelope and they're not going to open it."

German media have reported that Agassi, who was divorced from Brooke Shields in April 1999, and the never-married Graf are planning to wed in January after the birth of their child. Andre's mother says the love match between the two tennis stars "makes much more sense than Brooke and Andre. Not that I didn't like Brooke," she adds, "but Steffi and Andre have more in common."