Naomi Osaka opts out of WTA finals due to shoulder injury

The Japanese world number three was due to face Australia’s Ash Barty later in the day.

Naomi Osaka parts way with her coach Jermain Jenkins

Jenkins had become the coach of Osaka after the Australian Open this year.

Djokovic heads the men's field with Spain's Rafael Nadal seeded second. In the women's draw, Osaka is ranked no 1 Serena Williams is no 8.

Osaka is adamant she will defend her title, saying she would most likely play in New York even against her doctor’s wishes.

Osaka, who lost to Hsieh at the Miami Open in March, admitted a day earlier that the Taiwanese challenger was an unpredictable opponent.

Serena defeated Ekaterina Alexandrova to reach the quarters, while defeat Polish teen Iga Swiatek.

Defending champion Simona Halep, Osaka’s potential semi-final opponent, lost her way in the second set against Australian Ajla Tomljanovic.

The evergreen Federer did not drop a game in a dominant first set but was dragged into an intense battle by an inspired Monfils.

Ferrer, 37, announced earlier in the year he would end his 19-year career at the Madrid Open.

The 33-year-old Hsieh came from a set down to prevail 4-6 7-6(4) 6-3 as her unorthodox style frustrated, and wore down Osaka.