The South American tennis star had to withdraw from his quarterfinals match against Croatia's Borna Coric at the Masters 1000 final in Shanghai last Thursday.
Though Sevastova threatened late in the second set, Wozniacki broke in the ninth game to wrap up the title, the 30th of her career.
The Argentine top seed has been suffering with a cold in Beijing, but it was not immediately clear how much that had hindered his performance.
The Argentine top seed Del Potro will face either Britain's fifth seed Kyle Edmund or unseeded Nikoloz Basilashvili of Georgia in the final.
Significantly, victory means that Del Potro has qualified for the season-ending ATP Finals for the first time since 2013, joining Rafael Nadal, Roger Federer and Novak Djokovic in London in November.
The man fondly known as "Delpo" is ranked fourth in the world behind that trio and while there has never been any doubt about his ability, his career has been blighted by wrist injuries.
The 30-year-old is the favourite in Beijing with reigning champion and world number one Rafael Nadal out injured and Novak Djokovic and Roger Federer also absent.
Djokovic clinched his third US Open title on Sunday with a 6-3, 7-6 (7/4), 6-3 triumph over del Potro, taking him level with Pete Sampras's mark of 14 Grand Slams.
Novak Djokovic clinched his third US Open title on Sunday with a 6-3, 7-6 (7/4), 6-3 triumph over Juan Martin del Potro, taking him level with Pete Sampras's mark of 14 Grand Slams.
Djokovic can equal Pete Sampras's record of 14 Grand Slam titles when he faces Juan Martin del Potro in Sunday's final where friendship will be put to one side in pursuit of unfinished business.
Even his rival for the US Open title, 13-time Grand Slam champion Novak Djokovic, has a hard-time rooting against Del Potro, the 2009 US Open winner who has painstakingly rebuilt his career.
Djokovic will start the heavy favourite, boasting a 14-4 record over the Argentine who will be playing in just his second final at the Slams.
A dozen friends from del Potro's hometown of Tandil have gained fame in Flushing Meadows, their serenade of "Ole, ole, ole -- Del-Po, Del-Po" an infectious accompaniment to his every match.
The third-seeded Argentine has yet to drop a set in New York but was pushed hard by the left-hander.
Fifth seed Juan Martin del Potro overcame service troubles to beat Benoit Paire on Court Two and reach the Wimbledon fourth round.