But never before has Matsuzaka pitched to Matsui in a competitive situation. That will soon change.
Although Matsui and Matsuzaka haven't played together since 2004, clearly, the personal ties are still very much intact.
"We played together with Seibu, and he was always somebody that I respected tremendously, and he was someone who I always wanted to follow in his footsteps," Matsuzaka said.
Matsuzaka expressed as much when the two met for dinner after he signed his lucrative deal with the Red Sox.
"We just told each other that we should both do our best this year," Matsuzaka said.
With World Series implications on the line, it's safe to say both players have better things to think about than their long-running friendship and history together. The Red Sox are attempting to take a commanding 3-0 lead over the Rockies in the best-of-seven series, and Colorado, happy to be home again, has to make sure that the hole it dug itself into in Boston doesn't get any deeper.
Still, Matsui offered a small glimpse as to how he'll feel when he steps to the plate to face his former teammate for the first time.
"Personally, I cannot wait to face him," Matsui said with a faint smile. "But as a team, we have to be ready for the third game and we have to win it."
Three Japanese players are participating in the World Series -- Matsuzaka, Matsui and Hideki Okajima, who wowed the viewing public with his dominant 2 1/3 innings of relief for the Red Sox during Game 2 in Boston on Thursday night.
The strong Japanese presence in this year's Fall Classic has created an elevated level of excitement among Japanese fans, and it has also forced the myriad of Japanese media to work overtime. While Matsuzaka has carried his star power throughout the season, Matsui and his Rockies club came out of nowhere not long ago to burst onto the playoff scene. In that respect, the Japanese media corps was somewhat taken off guard.
Although they follow every Japanese player closely, having so many storylines during this World Series has increased the load even further. The general consensus is that Saturday's Matsuzaka-Matsui matchup is the most hyped story in recent memory.
"I didn't expect that matchup," said Hideaki Yonezuwa of the Fuji Evening News. "Nobody expected this.
"There are a bunch of Kaz fans in Japan who expect Kaz to play well and beat Matsuzaka. Of course, Matsuzaka fans want to see good pitching. He struggled for half of the season. It's pretty exciting, especially since they used to be teammates."
Hideki Okuda of Sports Nippon newspapers hopes Matsuzaka -- who, despite his 15 regular-season wins, was somewhat of a disappointment in the eyes of the Japanese media -- will throw seven or so strong innings, but not at the expense of Matsui.
"I don't think we want to see Daisuke strike out Kaz two or three times," Okuda said.
Regardless of the outcome, the Japanese media contingent is clearly proud of how well their country is being represented during this year's postseason.
"We are so excited," Okuda said. "It's a great story about Kaz Matsui. He struggled with the Mets, but now he's loved by Rockies fans and by the team. Finally, he can show his stuff to the people in the United States. With Daisuke, he's always been a star. We're proud of them all."