Serena Williams says she lives in fear of blood clots, a condition that surfaced during a harrowing postnatal ordeal in September when she almost died giving birth to her first child.

In an op-ed piece she wrote for CNN on Tuesday, the tennis legend lifted the lid on her near-death experience while giving birth to daughter, Olympia, after getting blood clots in her lungs.

"I almost died after giving birth to my daughter," Williams said.

The 23-time Grand Slam champion Williams said she had to have an emergency Cesarean section surgery after her heart rate plummeted dramatically during contractions. The surgery was successful and before she knew it she was holding the newborn.

"But what followed just 24 hours after giving birth were six days of uncertainty," she said.

In a Vogue magazine interview in January, Williams said that during her postnatal ordeal she suffered a pulmonary embolism -- when blood clots block one or more arteries in the lungs.

But this was not the first time the 36-year-old Williams has had a scrape with death from blood clots. In 2011, she spent nearly 12 months incapacitated after a cut on her foot from a piece of broken glass at a Munich restaurant led to a pulmonary embolism.

"Because of my medical history with this problem I live in fear of this situation," the American said Tuesday.

Williams said that while recovering in the hospital, one day after the emergency Cesarean, she felt short of breath and after some convincing on Williams' part, the hospital staff finally sent for a CT scan and then put her on a life-saving 

Wound popped open

But her ordeal wasn't over. She started coughing so much from the blood clots that her Cesarean wound popped open.

"I returned to surgery where the doctors found a large hematoma in my abdomen. Then I returned to the operating room for a procedure that prevents clots from travelling to my lungs.

"When I finally made it home to my family I had to spend six weeks of motherhood in bed."

Williams praised the hospital Tuesday staff saying "if it weren't for their professional care, I wouldn't be here today". She did not reveal the name of the hospital in the CNN piece.

Her kind words though were in contrast to some sharp statements she made in the earlier Vogue article where she says she had to coax the hospital staff to send her for a CT scan and hook her up to an IV.

"I was like a Doppler? I told you, I need a CT scan and a heparin drip (blood thinner)," Williams told the magazine.

Williams said it was a complicated experience and despite the agonizing ups and downs she "considers herself fortunate".

Partly for surviving the ordeal and also because she can still live out her dream on the tennis court once she returns to competitive form.

The former world number one staged her long-awaited tennis comeback earlier this month by playing alongside her older sister, Venus, in a Federation Cup doubles match but it didn't go well.

Serena -- looking rusty and slow-footed -- lost 6-2, 6-3 to the unheralded Dutch pairing of Lesley Kerkhove and Demi Schuurs.