The Polish teenager began her day with a 6-2 7-5 victory over No.5 seed Svitolina, twice a winner of this tournament in Rome.
It was a match that was carried over from Saturday's (AEST) schedule after rain interrupted play, and Swiatek delivered a convincing performance.
That set up the semi-final clash with Gauff, the 17-year-old American competing at this stage of a WTA 1000 event for the first time, having been gifted her last-four place when Australia's Ash Barty abandoned their quarter-final match because of an arm injury on Saturday (AEST).
There was no sign of fatigue from Swiatek as the 19-year-old powered to a 7-6 (7-3) 6-3 victory in a battle of the leading teenage stars of the women's game, who faced each for the first time in what could be a long rivalry.
Swiatek said: "I felt really good today. It's a big difference between my previous matches and today. I was able to beat Elina, so that shows that my game is here, I'm ready for anyone. It gave me a real confidence boost."
The No.15 seed added: "It's always hard to play against a young superstar, especially when I'm not used to that because I was always the youngest one.
"I just tried to have a high level of energy, even though it was my second match."
Between her two matches, Swiatek would have learned it was Pliskova waiting in the final, as the No.9 seed from Czech Republic defeated Croatian Petra Martic 6-1 3-6 6-2 to keep up her remarkable record in the Italian capital.
Pliskova won the Rome title in 2019 and was runner-up to Simona Halep last year, but the 29-year-old former world number one has not been to a final since this event unusually took place in September of last year, having been delayed because of the pandemic.
Swiatek won as an outsider at Roland Garros in October, defying her world ranking which at the time was a lowly No.54, and in Pliskova will face a player who has yet to win a grand slam title, a glaring absence amid an otherwise stellar career.
Against Gauff, Swiatek made 25 winners and just 16 unforced errors, a healthy ratio, and took three of her four break-point opportunities.
She sealed victory on a third match point, a big backhand forcing a stretching Gauff to chip beyond the baseline.
Gauff hit seven aces but also five double faults and had 33 winners and 32 unforced errors in the contest.