In contrast Williams, playing just her third tournament of a season that included a six-month injury layoff, had to rally after losing the opening set for the second match in a row.
She was helped by 12 aces, which were countered by just one double fault.
"What's working right now is my serve," said Williams, who took her lengthy injury layoff after an early exit at the Australian Open, and returned in Cincinnati last month. "It really was steady today. I only hit one double fault today, that's how I used to play."
But she was disappointed to have let the first set get away.
"I honestly should have won that first set," said Williams, who had three chances to break in the fourth game. "I missed a couple of easy shots, I let it slip away."
But she won the second with three breaks of serve, Shaughnessy double-faulting on Williams' third set point. Williams broke Shaughnessy to love in the eighth game of the third, but dropped her own serve for the first time all day when serving for the match in the ninth.
After saving one match point with an ace in the next game, Shaughnessy double-faulted on the next.
Williams, who fell out of the top 100 for the first time since 1997 during her injury absence this year, said she was pleased with her continued progress.
"I'm still on a steady track up," said Williams, whose semi-final appearance should put her around 80 in the rankings released on Monday."
She admitted that she still felt some intermittent pain in her left knee, and she played with her left ankle taped after twisting it in a fall during her match on Thursday night.
Shaughnessy said she didn't see any reason Williams couldn't make it all the way back to the summit of the game.
"She moves great," Shaughnessy said. "When she was in her prime she was probably a little more match fit, but she obviously has the skill and ability to bring it back where she wants to be."
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