What happened when Serena and Alexis forgot their wedding vows?

NEW ORLEANS -- Only one man stood between Serena Williams and Alexis Ohanian when they got married, and that man was Mackie Shilstone.  Call him "Minister Mackie."

Shilstone is an internationally-known fitness coach based in New Orleans, and he's trained several elite athletes.  Willams sought him out nine and a half years ago.

She flew to New Orleans to meet him, and Shilstone told her that he never takes on a client unless they become friends first.  They clicked right away.

Since then, Shilstone has been Williams' "tactical trainer,"  pushing the superstar tennis player through the kind of workouts that would make anyone less than a champion give up.  He's traveled to her matches around the world, and he and his wife Sandy have become Williams' close friends.

So when Williams and Ohanian decided to get married, Sandy (who's the former head of the New Orleans Tourism and Marketing Commission) persuaded Williams to have the wedding in New Orleans. It didn't take much to convince her, says Sandy, because "Serena loves it here."

Then, Williams had a request of Mackie.  She wanted him-- the man who's been with her through nearly a decade filled with victories and a few defeats-- to be the man who would officiate at the wedding.  Shilstone went through a short procedure to be certified to do it, and that's how he wound up "coaching" Williams and Ohanian through their wedding.

Watch our exclusive report, above, to see how he saved the day!