Joint is a talent at age 19, but she was blown away by the six-time Grand Slam champion, Swiatek, teaching her a lesson in a 6-1, 6-1 victory in 58 minutes.
The 24-year-old Swiatek will be among the favourites when the Australian Open starts on January 18th, and this showed why as she won a third match in a row.
Later, Alex de Minaur defeated Hubert Hurkacz 6-4, 4-6, 6-4 in the men's singles to level the quarter-final at 1-1 and give the hosts hope.
However, Poland won the deciding mixed doubles to set up a last-four showdown with reigning champions the United States in a repeat of last year's final.
That tees up Swiatek against world number four Coco Gauff.
The other semi-final on Saturday is between Belgium and Switzerland.
The United Cup is a mixed-teams competition featuring 18 countries and serves as a build-up to the Australian Open.