Novak Djokovic will return to action following his French Open disappointment in Vienna this month after accepting a wildcard.
Djokovic has not played Vienna since 2007 when he beat Stan Wawrinka in the final to win the competition.
He missed the initial cut-off to sign-up for the tournament, but he has taken a wildcard, putting him in a position to potentially all-but seal the year-end world number one.
Rafael Nadal needs an exceptionally strong end of the year to deny Djokovic, who also has the Paris Masters and ATP Finals to come.
However, since Djokovic has no points to defend in Vienna, anything he does win will be added to his total instead.
Djokovic joining the the field makes an exceptionally strong field even stronger.
World number three and current US Open champion Dominic Thiem will play, as will Daniil Medvedev.
World number five Stefanos Tsitsipas is also in the draw, although the injury he sustained against Djokovic in the French Open semi-final may ultimately rule him out.
Other names in the tournament include Gael Monfils, Andrey Rublev, Denis Shapovalov, Diego Schwartzman, Matteo Berrettini, John Isner, Milos Raonic, and Stan Wawrinka.
If Novak Djokovic is crowned year-end world number one, he will equal Pete Sampras' all-time record of achieving it six times.
Vienna, an ATP500 competition, is due to get underway on October 26.
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