In December 2000, Matsui made a return to pro wrestling at the Inoki Bom-Ba-Ye event, teaming with The Great Sasuke to defeat Akira Shoji and Caol Uno.
He also competed in Battlarts, beating Ikuto Hidaka, and in New Japan Pro-Wrestling, wrestling Kendo Kashin in a losing effort for the IWGP Junior Heavyweight Championship.
Matsui transitioned to mixed martial arts from pro wrestling and trained at the Takada Dojo for much of his career along with Kazushi Sakuraba.
In an entertaining fight, Matsui fended off increasingly difficult submission attempts from the Brazilian jiu-jitsu specialist and struck when possible, including a notable haymaker exchange at the end of the bout.
Matsui fought again at PRIDE 7 against Dutch kickboxer "Dirty" Bob Schrijber, in a fight in which he performed impressively, holding his own against the striker and taking advantage of his superior wrestling skills.
Matsui continued with renovated aggressivity, performing a takedown attempt which made Schrijber fall through the ropes and brutally land on a table ringside.
He ultimately lost in a unanimous decision, but he passed to history as one of the few opponents Silva wasn't able to finish, including Matsui's own partner Kazushi Sakuraba.
However, Matsui then fought against Ukrainian kickboxer Igor Vovchanchyn at PRIDE 9, and he was finished the first time in his career after the referee called a stop to the bout at 5:03 into the first round by an eye cut.
The Japanese had his back taken with a body triangle, but he recovered from the knee and started countering Pele's submission attempts through sheer heart, driving the Brazilian against the turnbuckle in full runs to fend off rear-naked chokes and dropping him on his head to escape from armbars.
Frustrated by his opponent's resistance, Pele landed a series of illegal elbows to the back of Matsui, resulting in a yellow card from the referee which attracted heated words.
[2] Although Daijiro had challenged Wanderlei Silva to a rematch, his next opponent ended up being Ebenezer Fontes Braga at PRIDE 15 and Matsui lost via unanimous decision.
Matsui made his debut for the England-based Cage Rage organization a year later against former Alex Reid and fought to a draw.