Chip giant Intel is seeking to overthrow a $1.33 billion fine which the EU antitrust regulators forced upon it.
Intel told the General Court in Luxembourg that commissioners relied on "profoundly inadequate" evidence in their case against Intel.
It wants a panel of five judges to over turn the fine, and is asking the General Court in Luxembourg, Europe's second highest.
The Commission was legally allowed to take 10 percent of Intel's 2008 turnover, and the fine, while huge, was only 4.15 percent.
Intel's lawyer Nicholas Green told the court that the Commission did not have sufficient evidence of any wrongdoing by Intel and relied too much on comments by the company's customers.
According to Reuters, he said that that the quality of evidence relied on by the Commission is profoundly inadequate and the analysis is "hopelessly and irretrievably defective".
But the lawyers for the Commission said this was at the core of Intel's illegal business practice.
Commission lawyer Nicholas Khan told judges that Intel's rebates were designed to tie customers and put competitors in an unfavourable position.
Intel carefully camouflaged its anti-competitive practices with clever wording in its contracts, he said.
The European Ombudsman has censured the Commission for procedural errors in its investigation. He said that the regulator had failed to make a proper note of a meeting with Dell during its probe.
Intel was also backed by the Association for Competitive Technology which represents more than 3,000 small software developers and information technology firms.
However the French consumer organisation Que choisir is backing the Commission.
Once the General Court has ruled, the loser is expected appeal to the highest court, the EU Court of Justice.