And now talks might get serious about them meeting in the ring.
The fighters were both sitting courtside, directly across from one another, at a Miami Heat game on Tuesday night. Mayweather went over to Pacquiao at halftime, the men shook hands and chatted briefly, then exchanged phone numbers.
"He gave his number to me and said we will communicate with each other," said Pacquiao, adding that he had never before met Mayweather in person.
Asked to clarify if the exchange meant the two would negotiate directly about meeting in the ring, Pacquiao said that was the case.
Mayweather did not respond to a question asked by an AP reporter. "Not now," one of his security guards said, as Mayweather fiddled with his phone.
A bout between the two would almost certainly be the richest ever in boxing, potentially grossing as much as $250 million. Mayweather's take alone could exceed $100 million.
Mayweather said last month that he's ready to make the fight happen May 2. Asked Tuesday if that date can finally be the one, Pacquiao — who has said it's time to give boxing fans what they want — nodded and said "yes."
When Mayweather said he wanted the May 2 fight to happen, he said Pacquiao promoter Bob Arum had been the main obstacle for it not coming together in the last five years. Mayweather repeated his old charges about Pacquiao not wanting to do blood tests prior to the fight for it not happening five years ago.
He also has no doubt that he could beat Pacquiao.
"I know that he's not on my level," Mayweather said last month.
Pacquiao has a similar confidence, saying on Twitter recently that "I can easily beat @FloydMayweather, I believe that."
It appears to be happenstance that Mayweather and Pacquiao were together on Tuesday.
Pacquiao has been in Miami for several days, after serving as one of the judges in the Miss Universe pageant at Florida International University. He was supposed to leave Tuesday night but said his flight was cancelled — as many were in South Florida because of the crippling effects felt nationwide by the havoc a winter storm caused in the New York and Boston areas.
So Pacquiao decided to come to the Heat-Milwaukee Bucks game. He's a noted basketball fan and has a good relationship with Heat coach Erik Spoelstra, who is of Filipino descent.
Mayweather has been a regular courtside attendee at Heat games for several years.
And it just so happened they picked the same game. Pacquiao said he didn't expect to see Mayweather — who had a seat just a few feet from Spoelstra's spot on the side of the court where the Heat have their bench.
Spoelstra met with Pacquiao before the game and said he was "totally shocked" to see Mayweather on the same night. If the fight happens May 2, it would occur somewhere around the end of the first round of the Eastern Conference playoffs.
"I hope we're extremely busy at that time, May 2," Spoelstra said, "and I hope they're very busy as well."
Mayweather, the WBA and WBC welterweight champion, hasn't lost in 47 fights. Pacquiao, the WBO welterweight titleholder, is 57-5-2 in his career.
Even the Heat in-game marketing arm weighed in on the buzz surrounding the potential fight. They put photos of both fighters on their video screens during a first-half stoppage in play, with the caption "Coming in 2015?"
There's no sure answer — yet.