Serena Williams fled to the panic room in her Los Angeles mansion when she mistook a sports association drug tester for a dangerous intruder.
The tennis legend's assistant called 911 around 6:00am local time last Wednesday and told the emergency operator that a prowler had been spotted at the luxurious property, TMZ reported.
Williams retreated to her panic room in a bid to protect herself.
But the intruder turned out to be a random drug tester who stopped by unannounced for a urine sample.
Cops responded to the 911 call but quickly left the property when the misunderstanding was discovered.
It was not known whether Williams submitted to the drug test.