Apple Inc filed a $1 billion lawsuit against supplier Qualcomm Inc on Friday, days after the U.S. government filed a lawsuit that accused the chip maker of resorting to anticompetitive tactics to maintain a monopoly over key semiconductors in mobile phones. Qualcomm is a major supplier to both Apple and Samsung Electronics Co Ltd for "modem" chips that connect phones to wireless networks. The two companies together accounted for 40 percent of Qualcomm's $23.5 billion in revenue in its most recent fiscal year.