"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", said the world No. 2, whose victory is bound to boost her confidence ahead of her title defence at the Oct 21-28 WTA Finals in Singapore.
"By the third match, I thought I played really great tennis".
"Having done everything, all of that, it's pretty fantastic".
"I'm just happy to have reached 30 today, I'm going to enjoy that", said Wozniacki, a former number one.
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.
"Playing against Donna, I had some problems with my stomach muscle and never expected to go further".
In the final match, Wozniacki held the opening set with a smash.
"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 two was better in the big moments against Sevastova, saving seven out of eight breakpoints against the Latvian, who racked up 35 unforced errors.
Nishikori corrected issues with his own serve in the second set but still could not have an impact on Medvedev's service games, winning only one point on the Russian's serve during the entirety of the second set.