World number one Swiatek, a four-time champion at the French Open, clinched her 25th win on the Roland Garros clay courts when Collins was later forced to retire in the final set with an injury.
There was a worrying moment in the opening game of the third set when the powerful Collins unleashed a fierce backhand into the body of the Pole as she approached the net.
Swiatek took evasive action but was still struck a painful blow on the chest. She crouched on the ground, catching her breath before continuing.
Collins immediately crossed to Swiatek's side of the net to apologise.
Top seed Swiatek swept the first set 6-1 before Collins levelled the quarter-final by claiming the second set 6-2.
Collins's challenge unravelled when she required an off-court injury time-out after three games of the final set.
Two games later and with tears in her eyes, the American retired with injury.
Swiatek, the first Polish player to reach the semi-finals at an Olympic tennis event, will face China's Zheng Qinwen for a place in the gold medal match.
