Following up her impressive win over Wimbledon finalist Jasmine Paolini, the Slovakian brushed aside the All England Club champion 6-4 6-2 in just under 90 minutes at Roland-Garros.
Schmiedlova raced into a 3-0 lead and dug deep to hold off Krejcikova's fightback when she clawed it back to 4-4, getting a vital break in the final game to take the first set.
It was a much better start in the second by the Czech as she got a first-game break, but Schmiedlova put together a remarkable six-game winning run to put the match beyond any doubt.
She will face either Croatia's Donna Vekic or Ukraine's Marta Kostyuk in the final four.
Data Debrief: Schmiedlova loves an upset
Schmiedlova (world no. 67) is the lowest-ranked women's singles semi-finalist at the Olympics since tennis returned to the summer program in 1988 - Na Li (#42) in Beijing 2008 was the previous lowest.
She had to come from behind to edge out Paolini, but after a dominant performance in which she had no double faults, she will be backing herself to cause more problems in the next round.
The world number one is the firm favourite to win gold at Roland-Garros, where she has won four editions of the French Open, including the last three in a row.
Swiatek and Begu traded breaks at the outset of their clash on Court Philippe-Chatrier, but the Pole continued to put her opponent under pressure when receiving and back-to-back breaks took the opener away from Begu.
The second set was more of a slog for Swiatek and a break in the seventh game had Begu serving to level things up, but Swiatek found a second wind to break back and only dropped one further point from there, winning 6-2 7-5.
Swiatek will face Argentina's Nadia Podoroska or home favourite Diane Parry in the second round, with the likes of Danielle Collins, Jessica Pegula, Naomi Osaka and Angelique Kerber also on her side of the draw.
On the other side of the bracket, Jasmine Paolini – the runner-up at both the French Open and Wimbledon this year – advanced with a 7-5 6-3 win over Ana Bogdan.
Data Debrief: Twenty-two and counting for Swiatek
Swiatek has now won a remarkable 22 successive matches on the courts of Roland-Garros, last losing at the venue to Maria Sakkari in the 2021 French Open quarter-finals.
In all clay-court tournaments, she has won her last 20 matches, also capturing titles in Madrid and Rome earlier this year.
