By TMZ Sports

Boxing superstar Saul "Canelo" Alvarez is in the middle of a nasty lawsuit filed by another Mexican fighter -- who's suing Canelo for $9 million over a street fight in Mexico back in 2011 ... TMZ Sports has learned.

The man behind the lawsuit is Ulises "Archie" Solis -- a former IBF light flyweight champ -- who claims Alvarez beat him up during an altercation in Guadalajara.

According to the suit, Alvarez punched Solis in the face so hard -- he knocked out a tooth and fractured Solis' jaw.

As a result, Solis claims he was stripped of his IBF championship belt because he couldn't defend his title due to the jaw injury which required multiple surgeries.

Solis claims he was simply outsized in the fight -- with Canelo coming in a 5'9", 160 lbs and Solis at 5'3", 108 lbs.

In his suit, Solis claims he suffered more than $4 million in damages -- plus he wants another $5 mil in punitives.

We've learned trial proceedings have begun -- jury selection started on Monday -- and the trial could wrap by the end of the month.