The tennis star uploaded two Instagram photos on Saturday showing her wearing a leopard-print swimsuit while posing with a bridal party dressed in all white.
"Wedding crasher!! Congrats," she captioned the first picture, adding another post with, "Bikini wedding crasher!"
"During the wedding, a few people realized that Serena Williams was at the beach," Sam tells PEOPLE. "I wasn't surprised, being that they're celebrities, that she was staying in South Beach Miami anyway. I didn't think anything about taking a picture; I was more focused on, you know, our wedding and vows. Fast forward to the end of the wedding, we're proceeding out, and a lot of the people said, 'Call her over …'
Adds the groom: "I told my friend Eddie, 'Eddie, go and see if she wouldn't mind taking a picture.' So Eddie went and asked. He said she was ecstatic and would be happy to. She was even asking, 'Let me take a picture with the bride and groom!' "
Ruth, who says the surprise encounter made her big day all the more special, explains: "She said congratulations, and she gave me a hug – that was the first thing she did. That was when she was like, 'Let's take pictures!' Everyone was so ecstatic, they kept running up. But we were able to get pictures, which was a great thing."
Perhaps Williams was still basking in the bridal bliss after watching pal Kim Kardashian walk down the aisle with Kanye West over Memorial Day weekend.
The 32-year-old was in town with fellow pro Caroline Wozniacki to watch the Miami Heat defeat the Indiana Pacers during the NBA's Eastern Conference finals on Friday night, even posing with player Greg Oden and the team's shiny new trophy.