The 21-year-old struck a staggering 35 winners to just 12 unforced errors in a dominant performance. Barty, seeded ninth, posted her first victory over Kerber after losing their only previous meeting in Brisbane at the start of the season.
"It was tough group for me because I hadn't actually beaten either of those girls," Barty, who beat Russian Anastasia Pavlyuchenkova in her first group match, told reporters.
"It's nice to sort of get that revenge this week, but I felt like I served great all week and put myself in a good position in my service games to then be super-aggressive and nice and clear when returning."
Sevastova, making her tournament debut, posted a perfect record in Camellia Group with a 6-3 6-4 win over Barbora Strycova of Czech Republic.
The Latvian came back from 2-0 down in the opening set, winning 20 of the next 24 points to take the first set, and later claimed a break in the seventh game of the second set to earn the victory.
"I feel it's my best Asian tournament in this year, so I feel great in Zhuhai," Sevastova said.
"I haven't been playing so well in China, in Asia, particularly, but here somehow it suits my game, I think, the court, the balls."
Later in the day, American CoCo Vandeweghe also progressed to the final four with a 6-3 6-2 win over Russian Elena Vesnina, who was struggling with a back injury throughout the match.
(Reporting by Hardik Vyas in Bengaluru; editing by Emelia Sithole-Matarise)
