World number one Williams, who is chasing a fourth consecutive tour title, will face Victoria Azarenka in Sunday's final after the third seed beat last year's French Open finalist Sara Errani 6-0 7-5.
Williams's unbeaten run has included title successes in Miami, Charleston and Madrid and she is chasing a first Rome title since 2002, the year she won her only French Open.
"I love the clay. I haven't had many wins at Roland Garros but I do love it," Williams said.
"I was unhappy with some things today. I want to do well here as well as (in Paris)."
Williams' opponent was playing her first top-level semi-final and was bidding to become the first qualifier to reach the final in Rome.
Halep scored an early break of serve for a 2-1 lead in the first set but then wilted, winning just one more game as 15-times grand slam singles champion Williams steamrollered her way into the final.
- Sports & Recreation
- Serena Williams
- Simona Halep
- Victoria Azarenka