*In the new issue of People magazine, Serena Williams admits she was terrified by the medical emergency that sent her to the hospital last week.

“I could have died,” the 29-year-old says of the pulmonary embolism and subsequent hematoma that she suffered.

Not grasping the reality of the situation at first, Williams says she felt shock and disbelief. “This can’t be happening to me,” she recalls thinking.

But it was happening – and it was serious. Pulmonary embolism is a blood clot in the lungs, and a hematoma occurs when blood leaks out of the vessels and gathers in one spot.

The hematoma, in particular, was unnerving for Williams, who didn’t know what was happening. “I started panicking,” she says. “I thought I had a tumor.”

The hematoma was successfully removed by surgery, and the 13-time Grand Slam singles champion is currently taking blood thinner medication and things one day at a time.

As previously reported, she’s planning a return to the court in July for the Washington Kastles of World Team Tennis.

“I like having a challenge,” she says. “And this will be my biggest challenge yet.”