Serena Williams, in search for a record-tying 24th title, defeated Serbia's Nina Stojanovic in straight sets 6-3, 6-0 in a contest that lasted one hour and nine minutes.
Bianca Andreescu, who did not defend her 2019 US Open title in New York this year, has not played on tour since the WTA Finals in Shenzhen last year.
World No.1 Ashleigh Barty, Svetlana Kuznetsova, Elina Svitolina and Kiki Bertens had already decided against playing in the US Open this year.
On September 8, the teenager defeated former world No.1 Williams 6-3, 7-5 in the summit clash at the Arthur Ashe Stadium.
Elina Svitolina dominated the second set from the word go and broke Karolina Pliskova in the very first game to eventually win first match of WTA Finals.
Osaka will now face US Open champion Bianca Andreescu, whereas Tsitsipus will face world number 19 John Isner.
The Canadian teen clinched the first set 6-3. Although Serena Williams put up a better show in the second set, Andreescu eventually bagged the set 7-5.
At the Arthur Ashe Stadium in the US Open finals, Williams was denied a record-equaling 24th Slam title as the 19-year-old Andreescu on Saturday became the youngest Grand Slam winner since Maria Sharapova at the 2006 US Open.
Andreescu will now take on No 8 seed and six-time US Open titlist Serena Williams in Saturday’s championship match.
Andreescu, 19, who clinched her second professional title at the Women's Tennis Association Premier 5 event in Toronto, needed just one hour and 55 minutes to reach her first Grand Slam quarterfinals.
