Serena Williams is receiving daily blood thinning injections to get rid of the clots that are still in her lungs, the former world number one revealed on Wednesday.

In her first public appearance since she underwent emergency treatment at a Los Angeles hospital for a pulmonary embolism, Williams described the experience as "the scariest moment in my life" but said she was on the road to recovery and hoping to make comeback.

"I had swelling in my leg which is a tell tale sign of embolism and I could not breath," Willaims told the Today show on Wednesday.

"I remember thinking, I'm walking but I cannot breathe, that forced me to the emergency room."

Williams said she had no idea of how ill she really was until doctors discovered the life-threatening clots during a CAT scan.

"Mine went from my leg to my lung. It travelled fast," she said.

"I still have several clots in my lung and they are still there. They have to eventually dissolve. I'm taking it a day at a time."

Williams has not played competitively since winning last year's Wimbledon championship in July. Shortly after the victory she cut her foot on broken glass at a restaurant in Germany.

The American has had two operations on her foot since then but recently developed a hematoma, which she believes was caused by the combination of her prolonged break and frequent flights.

"Because I'm on blood thinners and on the injections...I must have hit something," she said.

"Usually, your blood naturally clots around it (but) since I was on blood thinners it wasn't able to clot.

"So what started as a golf ball ended up being a grapefruit in my stomach."

Williams has won 13 Grand Slam singles titles during her glittering career but has slipped to 11th place on the world rankings during her extended absence.

The 29-year-old said she was recovering from the scare at her Californian home but was confident of regaining full fitness and returning to the courts later this year.

"I'm feeling better every day," she said.

"Luckily enough I was able to catch it soon enough that my career won't be affected.

"I love tennis and now, more than anything, I have so much to look forward to."