The Danish world number two was impressive all week in Beijing, never dropping a set and rarely looking troubled, sealing her spot in the WTA Finals Singapore in the process.
The Australian Open champion won 6-3 6-3 in 87 minutes in Beijing to claim her first title since Eastbourne in June.
Wozniacki had last won in Beijjng in 2010, following which she had become world No.1 for the first time in her career. Then I started believing that, you know, I'm really hitting my form, playing myself into shape.
Wozniacki said eight was her lucky number and, having won in Beijing eight years ago, she "felt it was my year".
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.
"I feel great being here".
Sevastova, 28, will find herself just outside the top 10 in the rankings after her run in the Chinese capital.
The first set, Wozniacki took due to the break in the sixth game.
"She didn't give me any presents, it was tough", said Sevastova, who retired in May 2013 because of persistent injuries, only to return in January 2015. The world number 20 made 35 unforced errors in the match, including 21 in the second set, to allow Wozniacki to close out the victory.
The 20th-ranked Latvian will face the victor of the semifinal between China's Wang Qiang and Caroline Wozniacki of Denmark. She sealed the win on her own serve and admits her game is now in a good place.
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