The second seed battled past her fellow American 6-7 (6/8), 6-4, 6-1 in a tense last-eight clash littered with a whopping 14 double-faults and 101 unforced errors.
Gauff will face the winner of the last quarter-final between Mirra Andreeva and world number 361 Lois Boisson for a place in her second French Open final on Thursday.
“It means a lot, especially getting through this tough match today, it wasn’t an easy match and I’m very happy to get through it,” she said.
“I’m so excited to be in the semi-finals back here again and have a lot more work to do. Just savour this one today and then next one tomorrow.”
Keys came through a tense first set in a tie-break after blowing a 4-1 lead with a double-break, but Gauff upped her level enough to fight back.
The 21-year-old was the runner-up to Iga Swiatek in a one-sided showpiece match in 2022.
Keys had been hoping to challenge for a second consecutive Grand Slam title, but failed to reach the semi-finals in Paris for a second time.
Gauff will be hoping to go at least one better than last year when she was beaten by Swiatek in the last four.
