SAN JOSE, Calif. — Escalating the world's largest computer chip maker's legal woes, the Federal Trade Commission has opened a formal probe into Intel Corp.'s sales tactics, a victory for its much smaller rival, Advanced Micro Devices Inc.

Intel disclosed yesterday that it has received a subpoena from the FTC for records about Intel's microprocessor sales, which dominate the world market with a roughly 80 percent share.

The FTC's two-year investigation had been considered "informal" until that point, and Intel, which is already fighting antitrust charges in the European Union and was fined this week by antitrust regulators in South Korea, said it had been cooperating.

By opening a formal investigation, Intel said, the FTC will be able to get access to documents revealing Intel's communications with certain customers — documents Intel couldn't voluntarily provide because of a protective order that is part of a sweeping antitrust lawsuit AMD filed in 2005 that isn't expected to go to trial until 2010.

The FTC's intensifying look at Intel's business practices is a result of AMD's long-running campaign to convince antitrust regulators around the world that its business has been hurt by Intel's aggressive tactics. AMD also said yesterday that it received a subpoena this week from the FTC — though the company said it is not a target of the investigation.

The two companies have been fighting for years over what AMD claims is Intel's intimidation of computer makers into striking exclusive deals for the chips they use in their new machines.