Swiatek began the day by hammering twice Grand Slam singles champion Barbora Krejcikova 6-0 6-3 in a quarter-final rescheduled from Friday due to persistent rain and the 24-year-old returned shortly afterwards to sweep aside Joint 6-0, 6-2.
"I'll just focus on myself and the goals that I had before and continue to do what I was doing throughout the tournament, because it's been working," Swiatek said before she found out that she would face second-seeded Russian Alexandrova on Sunday.
"The final is supposed to be the toughest and it always produces a different kind of stress, so I'm just happy that I've already played solid matches here."
Australian teenager Joint, who earlier downed Clara Tauson 6-0, 6-3, found herself in all kinds of trouble against an ultra-focused Swiatek and tamely surrendered the first set on Centre Court in only 25 minutes.
The top seed briefly wobbled in the second set to hand Joint the opportunity to break, but she quickly reset to go 3-1 up and never looked back from there as the 24-year-old wrapped up the contest with a powerful smash at the net.
Swiatek, whose rower father Tomasz represented Poland in the men's quadruple sculls event at the 1988 Seoul Olympic Games, is hoping to mark her first appearance in the South Korean capital with her third trophy of the season.
The world number two is also looking to close the gap on top-ranked Aryna Sabalenka, who pulled out of the China Open scheduled to begin in Beijing next week after sustaining a minor injury during her run to the US Open title.
Up next for six-time Grand Slam champion Swiatek is Alexandrova, who defeated Katerina Siniakova 6-4, 6-2 in the other last-four encounter.
Czech Siniakova earlier reached the semi-finals with a 6-1, 7-5 win over Suzan Lamens after Russian second seed Alexandrova advanced following a 6-2, 6-3 victory over Ella Seidel in other quarter-final matches postponed due to rain.