The 21-year-old American was a surprise winner over the third seeded Spaniard Garbine Muguruza. It will be the first all-American women's claycourt final in 14 years.
The last time two Americans faced off in the final of a WTA claycourt event was in 2002, when Williams beat her older sister, Venus, to win her maiden French Open at Roland Garros.
Williams, 34, said she was excited to see a new crop of Americans emerging from her shadow.
"Yeah, it's great. I think it will be wonderful because I feel like Madison is one of the players that really can be great and she has that potential. She's showing that on all surfaces," said Williams.
"We're going to have an all-American final on the clay. That's just great. It's really exciting."
As for her own form as she prepares to defend her French Open title in Paris from 22 May, she said: "I feel like everything is kind of coming together.
Williams favourite for Rome
Williams will be the favourite to lift the crown in Rome. She has beaten Keys in both of their previous meetings The first was in the semis at the 2015 Australian Open and the second came in the last 16 at the 2015 US Open.
"I played her pretty close the first set of the Australian Open," said Keys on the eve of the final. "So that definitely made me realise that I can stay in it.
"I think if you're able to weather the storm and stay with her then every once in a while you get one or two opportunities and it's all about taking those opportunities.
"I'm going to go out and do my best."
Williams said she expected a tough battle. "One thing I have learned is that Madison gets better every time and that's great because when you play people, especially when they're young like her and the future of American tennis, it's so good to see them get better.
"So for me it's just about trying to do the same thing, even though I'm a little older, just to get better and to be ready for her and to do the best that I can."