Austin Horse of the United States took the men's honour.
Reitzel, who also took the honours at the 2010 and 2012 editions in Guatemala and Chicago, shone on the hilly, cobbled Lausanne roads that she plies as a professional courier.
Fellow Swiss riders Renata Buccheli and Sabine Studer completed the podium.
New Yorker Horse, meanwhile, earned his first international award, beating Lausanne's Jonas Vuille and German courier Johannes Killisperger.
Reigning men's champion, Seattle rider Craig Etheridge, failed to add a third title to his haul. Like Reitzel, Etheridge had won both the 2012 and 2010 editions.
Launched 20 years ago, the championships honour the riding and delivery skills of cycle messengers — as well as their sub-culture, fashion and jargon, with categories such as "raddest dude".
The racing reflects the challenges of a messenger's regular working day, with competitors picking up and dropping off packages along a fixed course which includes checkpoints.
Besides speed, messenger competitions also test the participants' strategic ability to plan routes in order to ensure timely deliveries.
In the women's event, Reitzel notched up the required 70 deliveries in a total of three hours, two minutes.
In the men's competition, meanwhile, Horse clocked his 77 deliveries in a time of two hours, 52 minutes.
In total, 550 riders from 40 countries headed to Lausanne for the six-day championships, with 100 making the cut for Sunday's finals.
The 2014 edition is scheduled to take place in Mexico City.