Williams is the only player in either bracket with a seed higher than her world ranking. The 23-time Grand Slam champion is ranked No. 26 by the Women's Tennis Association.
"After balancing a variety of factors, including her return to competition following the birth of her daughter, her recent hard-court performance this summer and recognition of her achievements at the US Open, the USTA has moved Serena Williams' seeding nine spots, from 26 to 17," U.S. Open managing director of corporate communications Chris Widmaier said, according to USOpen.org.
"We feel this is the appropriate decision, one that recognizes Serena and is fair to the remaining seeded players."
Williams is a six-time U.S. Open champion, with her last victory coming in 2014. She posted photos on social media on Tuesday, with the simple caption "I'm back."
The 2018 U.S. Open begins on Monday at the USTA Billie Jean King National Tennis Center in New York City.