GOAT to GOAT: Djokovic, Nadal, Serena and Venus among best tributes to Federer

WATCH: In 2003, Roger Federer spoke to Tennis Channel about what his tennis future might hold.

The news of Roger Federer's imminent retirement from tennis reverberated inside and outside the tennis world this week, with a litany of stars from the past and present paying tribute. (Plus, one well-known American comedian.)

However, there was no one that tennis fans were more excited to hear from than the Swiss' peers in the 'GOAT' debate: Novak Djokovic, Rafael Nadal and Serena Williams.

They delivered. Each champion posted a heartfelt message on their respective social media channels this week.

Nadal was the first of the golden trio to weigh in on Twitter, saying in part: "Dear Roger, my friend and rival. I wish this day would have never come. It’s a sad day for me personally and for sports around the world."

Rounding out the tributes from the "Big 3," Djokovic added similar sentiments in an Instagram post: "Roger, it’s hard to see this day and put into words all that we’ve shared in this sport together. Over a decade of incredible moments and battles to think back on. Your career has set the tone for what it means to achieve excellence and lead with integrity and poise."

Federer will reunite with his two chief rivals for one last time next week at the Laver Cup, his last professional tournament. However, they'll all be on the same side of the net: The trio, along with Casper Ruud, Andy Murray, Stefanos Tsitsipas and alternate Matteo Berrettini, look to win Team Europe's fifth straight Laver Cup.

Federer and Serena Williams were champions at the same Grand Slam tournament six times, and they've now stepped away from tennis together. In her tribute, the 23-time Grand Slam singles champion drew parallels to their simultaneous two-plus decades at the top of the game, and their new evolutions.

"Our paths were always so similar, so much the same," she wrote. "You inspired countless millions and millions of people - including me - and we will never forget. I applaud you and look forward to all that you do in the future. Welcome to the retirement club. And thank you for being you."

And Venus Williams, who posted for the traditional Wimbledon champions' photo with Federer twice, summed it all up succinctly: "The greatest ever. Miss you already."