A judge at the US International Trade Commission has made an initial determination that HTC infringed two Apple patents.
If the judgment is made final, HTC could be banned from importing phones to the US.
It's the latest blow to Google's Android operating system, which is being attacked by competitors including Apple, Microsoft and Oracle.
The initial determination will now be reviewed by a larger panel of ITC judges, who can uphold or reject it.
The two patents appear to be fundamental to Android, according to Florian Mueller, a patent expert. "They are very likely to be infringed by code that is at the core of Android," he wrote in a blog post. The same patents are also at the heart of a dispute between Apple and Motorola, he said.
Apple could choose to license the technology to HTC, but Mueller thinks Apple won't do so unless HTC has patents that Apple needs in return.
Friday's ruling responds to one of two ITC complaints filed against HTC by Apple. HTC has also filed its own ITC complaint against Apple.
HTC said it didn't have access yet to the full opinion, and it sought to downplay the decision.
"Apple filed suit on 10 of its patents against HTC, but based on the judge's initial decision today only prevailed on 2 of those patents. HTC will vigorously fight these two remaining patents through an appeal before the ITC Commissioners who make the final decision," Grace Lei, general counsel for HTC, said in a statement.
"We are confident we have a strong case for the ITC appeals process and are fully prepared to defend ourselves using all means possible," she said.
HTC also noted that the ITC has ruled that Apple infringed patents assigned to S3Graphics, a company HTC is in the process of acquiring, as part of a separate investigation.
Android is being attacked from other sources as well. Microsoft appears to have been approaching Android handset makers requesting that they license its technology, and HTC is one of the biggest vendors to have complied so far.
Separately, Oracle is battling Google for allegedly infringing Oracle's Java patents and copyrights in Android. Apple has lodged complaints against other Android handset makers as well, including Samsung.