The 28-year-old, who won her first Grand Slam at the Australian Open at the start of this year, was imperious against the unseeded Sevastova.
Sunday's win was her second at the event in 10 years and it now takes her to 30 career titles on the WTA circuit.
"I feel great being here".
"Unfortunately, I have been playing through pain in my back this whole week", the world No. 6 said in a subsequent statement released by organizers.
"I wasn't able to play with that fire that I have in all my matches", Del Potro added, saying however that 34th-ranked Basilashvili "played unbelievable". With this victory, Wozniacki ameliorated her winning record to 5-0 against her opponent.
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Fognini, who was chasing a fourth title of the season, reportedly has an ankle injury. The time off allowed her body to mend and she returned to the sport in January 2015.
Sevastova edged a tight first set with a solitary break as Osaka's game showed signs of falling apart. "Having won again, eight is my lucky number and it is eight years ago I won here so I felt like it was my year", Wozniacki said on-court after her victory.
"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.
"Having done everything, all of that, it's pretty wonderful".
The world number two was better in the big moments against Sevastova, saving seven out of eight breakpoints against the Latvian, who racked up 35 unforced errors.
She sealed the win with the second of three match points on her own serve.