Serena Williams says she "almost died" after giving birth to her daughter, Alexis Olympia -- if not for a team of super doctors who removed a large hematoma from her abdomen.
The tennis star says her daughter was born by an emergency C-section on Sept. 1 after Olympia's heart rate plummeted during contractions.
The surgery went smoothly, Serena said on CNN.com ... "But what followed just 24 hours after giving birth were six days of uncertainty."
Williams says she suffered a pulmonary embolism -- which is when a blood clot blocks one or more arteries in her lungs.