The tester visited outside Williams' specified 'whereabouts window' and found her not at home.
No athlete is obliged to comply with a testing visit if it is not made during the 60-minute window for which the athlete identifies their whereabouts in advance each day.
The incident was made public after WTA chief Steve Simon was overheard leaving a message on Williams' answerphone at an airport.
Williams, preparing for the start of Wimbledon on Monday, admitted she was disappointed the story had gone public.
But the seven-time Wimbledon champion also made it clear she didn't expect to have been tested so often this year.
"It's unfortunate but I guess when you're in my position, you mention my name, people overhear things and they want to tell the press. It's something I've dealt with for a long time. That's okay," she told a press conference on Sunday.
"I actually thought the article was interesting. I didn't know I was tested so much more than everyone else. I do know I'm always getting tested all the time, no matter where I'm ranked.
"I didn't realise it was such a discrepancy, at least with the American players. It would be impossible for me not to feel some sort of way about that." Concerned at being tested more than her fellow American players, Williams revealed she had spoken to USADA chiefs.
The 36-year-old had been tested five times, compared to one or none from some of her compatriots.
Athletes returning from a long absence are often the subject of repeated drug tests, as they are seen as part of a higher-risk category than athletes in the flow of regular competition.
But, calling for equal treatment, the 23-time Grand Slam winner said: "I had a conversation with the lead guy with USADA. I think he was mentioned, as well.
"I sat on player council for 12 plus years. Normally it goes on ranking. I get it even if I'm No. 1. It goes on that number for ranking for 12 months. In January that 12 months for me would have been over.
"How is it I'm getting tested five times in June? It's only June, I've been tested five times. I'm okay with that.
"Literally verbatim I said, I'm going with that, as long as everyone is being treated equally. That's all I care about.
"I despise having people in our sport that aren't being honest. I'm totally okay with testing and I encourage it.
"What I want to know is everyone is getting tested, that we are really working to keep this sport clean.
"Tennis has given me so much. It's such an amazing sport. I feel like equality, that's all I've been preaching, it's all about equality.
"If that's testing everyone five times, let's do it. Let's be a part of it. It's just about being equal and not centering one person out.
"Just due to the numbers, it looks like I'm being pushed out. Just test everyone equally.
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