Serena Williams Congratulates Maria Sharapova on Tennis Hall of Fame Induction

Serena Williams and Maria Sharapova were rivals during their tennis careers, but they are now friends in retirement. 

And that friendship was on full display Saturday when Williams made an appearance as "a surprise inductee" during Sharapova's induction into the International Tennis Hall of Fame, per James H. Williams of USA Today.

Williams also took to Instagram and wrote the following congratulatory message:

"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. 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."

The rivalry between Williams and Sharapova was a rather one-sided one during their playing careers.

After all, Williams went 20-2 in their 22 head-to-head matches. She also went 8-1 when they faced each other at Grand Slam events and 3-1 in major finals. Sharapova did notch a victory at the Wimbledon final, but it was largely nothing but dominance from Williams when they faced each other.

That doesn't mean it came easy.

"There are only a few players in my career who challenged me to be the very best, every single time we stepped out on the court," Williams said of Sharapova. "Maria Sharapova was one of them. Whenever I saw her name next to mine in the draw, I made sure I practiced harder."

Sharapova echoed a similar sentiment in her acceptance speech

"It's a gift to have someone who motivates you to reach those heights. And I'll forever be grateful for bringing out the best in me," she said. "We both knew no other way than to fight our hearts out. ... We both hated to lose more than anything on this earth, and we both knew that the other was the biggest obstacle between ourselves and the trophy."

Despite the struggles against Williams, Sharapova was largely brilliant during her career.

She won 36 singles titles and completed the career Grand Slam in the process as a five-time major champion. She prevailed at Wimbledon in 2004, the U.S. Open in 2006, the Australian Open in 2008, and Roland Garros in 2012 and 2014.

Sharapova is one of 10 women to complete the career Grand Slam and will now forever be remembered as a legend of her sport.

Fittingly, she was inducted into the Hall of Fame by another one of those legends.