Sevastova, who eliminated US Open champion Naomi Osaka of Japan from the semifinal, did not make it easy and fought to win the final, but Wozniacki managed to snatch her 22nd trophy on hard courts.
Del Potro's defeat came as a shock for many, and was blamed for being a result of illness by the player, who had been suffering from a cold the past few days.
Wang's quarter-final win over Aryna Sabalenka was her 16th from her last 18 matches in her homeland, but she found herself under early pressure here and was broken in her opening service game when she netted a backhand.
"It was one of my best tournaments ever", said the runner-up.
Russian qualifier Daniil Medvedev stunned home favourite Kei Nishikori in the final to claim the Japan Open title after a comfortable 6-2 6-4 victory in Tokyo yesterday.
Evacuations ordered as Florida braces for Hurricane Michael
Monday, Michael's top sustained winds were around 85 miles per hour (140 kph) as it headed north at 12 miles per hour (19 kph). Forecasters said Michael could be near a very unsafe Category 4 by landfall, with further strengthening expected overnibght.
"I'm just happy to have reached 30 today, I'm going to enjoy that", said Wozniacki, a former number one. Having done everything, all of that, it's pretty awesome.
The 2018 Australian Open champion did not drop a set across five matches throughout the tournament, and in the final hit 12 winners and four out of eight break point chances to take out the match in 87 minutes. "She didn't give me any presents, it was tough", said Sevastova.
"I think I had a dream of that, but I don't know how realistic I really believed that was going to be".
Caroline Wozniacki won her third title of the year as she completed a 63 63 win over Anastasija Sevastova at ChinaOpen.
The 20th-ranked Latvian will face the victor of the semifinal between China's Wang Qiang and Caroline Wozniacki of Denmark.