WBO heavyweight champion Deontay Wilder and Olympic gold medal winner Alexander Povetkin battled in court on Tuesday, at a trial over a title bout that was called off after the Russian tested positive for a banned substance.

The boxers sat on opposing sides of a Manhattan federal courtroom as jurors heard their attorneys give opening statements in a trial focussed on whether Povetkin ingested meldonium after a World Anti-Doping Agency ban of the drug went into effect in January 2016.

That question has become key to duelling lawsuits the boxers have filed against each other over their scheduled fight for May 21, 2016 in Moscow, which was called off after Povetkin tested positive on April 27 for meldonium.

Judd Burstein, a lawyer for Wilder and promoter Lou DiBella, told jurors that positive urine test came after three negative ones earlier in April, which meant that Povetkin took the drug after passing the earlier tests.

"The only rational explanation for what transpired here is that Mr. Povetkin took meldonium in 2016," he said.

But Kent Yalowitz, a lawyer for Povetkin and promoter Andrei Ryabinskiy's company, said the Russian boxer had, like many other athletes, taken meldonium at a doctor's direction before the World Anti-Doping Agency had even announced plans to ban it.

"The evidence will show that meldonium can stay in the body for many months," he said.

The lawsuits came after the World Boxing Council announced on May 13 that Povetkin had tested positive for meldonium and subsequently announced the postponement of his bout with champion Wilder.

In June 2016, Wilder and DiBella sued Povetkin and Ryabinskiy's World of Boxing LLC, saying they were owed at least $5 million for the defendants' breach of a contract requiring Povetkin to be produced for the match.

Povetkin and World of Boxing soon after countersued, seeking $34.5 million for what they said was Wilder's own breach of contract for walking away from the fight and defamation for engaging in a "smear campaign."

The World Boxing Council in August announced that based on scientific and medical information it received, it was not possible to determine whether Povetkin ingested meldonium after Jan. 1, 2016, when it was officially banned.

In December, a bout between Povetkin and Bermane Stiverne was called off after the Russian tested positive for a different banned substance, ostarine.