More than half of Gauff's points came from the Australian Open champion soft mistakes and the world No.2 got away with a total of 10 double faults. Altogether there were over a 100 errors.
The 21-year-old will need to make dramatic adjustments when she faces either France's Lois Boisson or Russian sixth seed Mirra Andreeva for a place in Sunday's final.
Keys, who was looking to reach the last four at Roland Garros for the second time after 2018, bowed out with her 60th unforced error to end a forgettable contest.
"I'm happy to get through this match today, I have a lot more work to do, I'll be ready tomorrow," Gauff said.
"I changed something at 4-1 (in the first set). It's tough to play against her because she hits the ball so fast, so low, I was just trying to fight for every point ... to get the ball to the other side of the net."
Gauff, who reached the final here in 2022, is the youngest woman to claim 25 main-draw wins at Roland Garros since Martina Hingis (1995-2000).
The opening set was a scrappy affair, with the first three games going against serve.
Keys secured the first hold to lead 3-1 and broke again before Gauff clawed back a break and held for the first time to close the gap to 4-3.
In a flurry of breaks, Gauff levelled at 4-4 and held once more to edge ahead, putting the pressure on Keys, who saved a set point to force a tiebreak.
While the quality was patchy at best, the drama was undeniable, with momentum swinging wildly. In the end, Keys edged the tiebreak as Gauff double-faulted three times, handing the opener to her opponent.
Things did not get better in the second set.
"So many unforced errors," Gauff said on court, in one of the understatements of the season.
She levelled the contest as Keys looked out of sorts and cantered through the decider.
