NASCAR’s Kyle Busch Final Days Detailed in Death Certificate

NASCAR legend Kyle Busch had reportedly been battling bacterial pneumonia for "days to weeks" before his death ... and his condition allegedly spiraled fatally in just 24 hours once sepsis set in.

According to his death certificate, obtained by Us Weekly, Busch's pneumonia progressed into sepsis roughly one day before he shockingly died last Thursday.

From there, the sepsis reportedly caused dangerous clots to spread through his body ... before things took an even worse turn with severe bleeding that sent his body into shock.

As TMZ Sports first reported ... a 911 caller said Busch was coughing up blood and struggling to breathe before he was rushed to a North Carolina hospital after being found unresponsive inside a racing simulator.

The caller told dispatchers Busch was on the bathroom floor and felt like he was going to pass out.

Kyle's death came 11 days after a NASCAR Cup Series race in which he radioed to his crew and asked for a doctor to give him a "shot" after he finished the race.

Busch died at 41 years old ... leaving behind his wife Samantha and their two children, Brexton and Lennix.

No one will race the No. 8 car after his tragic death ... with Richard Childress Racing suspending the digit in his honor until Brexton is ready to join NASCAR.

The Busch family leaned on each other Sunday as thousands of NASCAR fans honored the racing legend during an emotional tribute before the Coca-Cola 600 at Charlotte Motor Speedway.