The 28-year-old Wozniacki, who won the China Open in 2010, was once more imperious against the 20th-ranked Sevastova, who briefly retired from tennis in May 2013 with injury.
Wozniacki is in the hunt for her third title of the year, following her Australian Open triumph in January and her victory at the Eastbourne International in June.
The 20-year-old Osaka required a medical break in the second set for what appeared to be lower back pain.
Recently-crowned US Open champion Naomi Osaka has pulled out of the ongoing Hong Kong Open citing a back injury.
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Osaka, who was attempting to reach a third straight final after NY and Tokyo's Pan Pacific Open, was unhappy with her behaviour in Friday's quarter-final victory over Zhang Shuai. "The physios did a good job and somehow I'm still hanging in there, serving OK", world number 20 Sevastova said. "I needed to really work it, to work hard, win the points".
"Playing against Juan, he was sick today for sure, I could see that", said the all-action Georgian.
The German spit out 31 unforced errors to 16 for her opponent, who notched 6 aces and 93 percent of her first service points overall to seal the win in just over one hour.
"It's a long way to get to the top 100".
The Argentine top seed Del Potro will face either Britain's fifth seed Kyle Edmund or unseeded Nikoloz Basilashvili of Georgia.