Prosecutors allege the pair sought to hide the Barcelona forward's earnings in offshore accounts, amounting to the defrauding of the Spanish government to the tune of €4.1 million ($4.6 million). They could face a jail term of up to 22 months if found guilty.
The trial opened Tuesday, but injuries prevented the Argentina star from appearing. He and his father, Jorge Horacio Messi -- who manages his financial affairs -- are only obliged to appear Thursday in order to testify.
Spanish sport newspaper Marca reported
that six witnesses, including principals from his former law firm, Juarez and Associates, and an expert have testified in court over the past two days that Messi did not have any knowledge of the contracts that he was signing, and that his father and brother were responsible for the documents.
"The mediator was always Jorge Messi or, occasionally, (his brother) Rodrigo Messi, but it was never Leo," the law firm's Inigo de Loyola Juarez testified.
Prosecutors filed charges in 2013. Messi has denied any wrongdoing, and in August of that year he and his father paid $6.75 million in back taxes plus interest.
Unpaid taxes on endorsements
The complaint centers on alleged tax fraud between 2007 and 2009. Prosecutors contend Messi and his father, aiming to lower their Spanish tax bill, sought to manage the player's lucrative income from image rights through shell companies set up overseas.
In June 2014, a Spanish state prosecutor asked the judge to drop the tax fraud charges against Messi but not his father. Later that year, a judge ruled that the case against both would proceed, despite the state prosecutor's request.
Messi, 28, has an annual salary from his club, FC Barcelona, of more than $20 million, plus endorsements from sponsors including Adidas and PepsiCo.
According to Forbes, he earned an estimated $73.8 million
"We have always fulfilled all our tax obligations, following the advice of our tax consultants, who will take care of clarifying this situation," Messi posted on his Facebook page when the charges were initially filed.
'Panama Papers' link
Messi was implicated in the so-called "Panama Papers" offshore leak in April 2016, according to reports.
FC Barcelona has promised to give Messi legal and financial support
, as the Argentina international considers whether to sue over the leak.
Messi, who arrived at Barcelona as a 13-year old in 2000 and gained Spanish citizenship in 2005, has won eight Spanish Primera Liga titles and three Champions League crowns during his time at the club. He is Barcelona's
and the league's, all-time leading scorer.
Meanwhile, his national team Argentina will back down from a threat to fly the team home from the U.S., where it is due to compete in the centenary Copa America. The country's FA is embroiled in a scandal involving broadcasting rights payments.