Williams' surprise appearance to induct Sharapova was "months in the making" and required the code name of another female icon.
Serena Williams said it best after she surprised the crowd by coming out to induct her former fierce rival, Maria Sharapova, into the International Tennis Hall of Fame.
"I know I'm probably the last person you would be expecting to see here tonight," Williams said in her speech. "Honestly, a few years ago I probably would've said the same thing."
Sharapova shared on "TODAY" on Monday that the special moment with Williams at the ceremony in Newport, Rhode Island, over the weekend was so top secret that Williams even had a code name when referring to her presence at the event.
It was only fitting that a legendary player known by her first name had the secret moniker of another icon known by one name.
"Her code name was 'Madonna' for a couple of months, which was very fun," Sharapova said. "She's such a great sport. I'm just humbled that she joined me onstage and introduced me. A true class act, and we've played so many tough matches against each other, so to share this memory after so many years was incredibly special."
Williams, 43, also paid tribute to Sharapova, 38, with a post on Instagram on Sunday, sharing photos from the event.
"Last night -- I flew to Rhode Island for Maria Sharapova. Maria's career as her dad put it yesterday -- started with me and last night ended with me inducting her into the tennis hall of fame," Williams wrote. "I'm probably the last person you expected to see standing here tonight. Honestly, a few years ago, I would've said the same thing. But years after getting to know her better -- we now call each other friends. And when a friend calls you answer. This is women supporting each other and respecting each others greatness. Congratulations @mariasharapova it is my honor -- as her former rival, her forever fan, and now, her forever friend -- to welcome Maria Sharapova into the International Tennis Hall of Fame."
Sharapova is one of only 10 women in history to win a career Grand Slam in singles by capturing titles at the Australian Open, U.S. Open, Wimbledon and the French Open.
Sharapova's first Grand Slam singles title came by beating Williams at Wimbledon in 2004 when Sharapova was still a teen. Sharapova then ascended to the No. 1 ranking in the world in 2005.
It was an incredible journey to the top for a player who moved to the U.S. from Russia with her father in 1994 with only $700 in savings and the goal of becoming a professional tennis player.
"My dad put it this way, and it was very sweet, he said our career started with watching her from Florida," Sharapova said about Williams on "TODAY."
"She used to practice with Venus right in front of my eyes as I was a little girl. So to then beat her at Wimbledon when I was 17 years old, and then to have this incredible rivalry, and then see her onstage introduce me was like a full-circle moment."
Williams had a decided upper hand in the rivalry, beating Sharapova 20 out of the 22 times they played.
"Let's be real. Maria and I were once the fiercest of rivals," Williams said in her speech. "We had our differences. To the world, we looked miles and miles apart, but the truth is we weren't. We wanted the exact same thing at the exact same time -- to be the very best. ... That's what made our rivalry so electric and so iconic."
The two played each other in four Grand Slam finals, the last coming in 2015 at the Australian Open. Williams won three of them.
"When the Tennis Hall of Fame asked us who would you like to introduce you onstage, she was the first person I thought of," Sharapova said. "And I said, 'I have to ask her.' And I immediately got my phone, I sent her a text message, and we spoke for about an hour."
"And I just said, 'You've come to mind, I have so much deep respect for you,'" she continued. "Our friendship has blossomed in the last few years. We've done a few engagements together, we've seen each other at all these events, and we just gossip all the time."
In her speech, Williams detailed her reaction after Sharapova reached out to her.
"A few months ago, Maria texted me that she needed to talk. ... When I called her, she asked me if I would introduce her into the Hall of Fame," Williams said. "Before she even finished her question, I said yes -- loud, immediately, with my whole chest. Because, of course, it's Maria."
Sharapova and Williams were able to keep her appearance at the induction ceremony under wraps despite Williams being one of the biggest stars in tennis history.
"I can't believe we were able to keep it a secret because it's been a couple months in the making," Sharapova said.
"And you heard that energy of her walking out onstage, and there was a lot of impact," she added. "I wanted that, I wanted her presence there."
Sharapova will receive her Hall of Fame ring at the U.S. Open on Monday at Arthur Ashe Stadium in New York City. She will be wearing a redesigned outfit inspired by the Nike dress she wore when she won the 2006 U.S. Open.
"I feel like I'm still in this joyful bubble, and I don't want it to burst," she said.