The 22-year-old Williams continued her quest for a fourth straight title here. The world No. 2, who has won her last two tournaments, has reached six consecutive WTA Tour finals, including the French Open and Wimbledon, where she lost to younger sister Serena.
One of four seeds who received first-round byes, Williams owns a 51-6 match record this season, including a 27-3 mark on hard courts. Three of her six titles have come on this surface.
Shaughnessy lost in the early rounds for her fifth straight event and has not reached a quarterfinal since June at Eastbourne. She fell to 1-5 lifetime against Williams.
Second seed Lindsay Davenport of the United States posted a 6-4, 7-6 (7-3) victory over Iroda Tulyaganova of Uzbekistan earlier Tuesday.
Davenport was runner-up twice in the last three years and is trying to win this tournament for the second time. Her first triumph came in 1997, the only year the tournament was held in Atlanta and the last time it took place outside New Haven.
Davenport improved to 13-3 after missing nearly nine months while recovering from knee surgery. She reached the semifinals at Stanford and San Diego before advancing to the final at Los Angeles two weeks ago.
In a first-round encounter, sixth seed Amelie Mauresmo of France, who captured her second title of the year in Canada on Sunday, continued her winning ways with a 6-2, 6-2 win over Russia's Tatiana Panova.
Both of Mauresmo's tournament victories this season have come on hard courts and she has reached the quarterfinals in 10 of 13 events this season.
In a pair of other first-round matches, Russia's Anastasia Myskina swept past Spain's Conchita Martinez, 6-1, 6-4, and Patty Schnyder of Switzerland ousted another Spaniard, Marta Marrero, 6-4, 6-1.
Early second-round action Tuesday saw seventh seed Daniela Hantuchova of Slovakia beat Jennifer Hopkins, 6-3, 6-4, while fellow American Laura Granville got past German qualifier Angelika Roesch, 7-5, 6-2.