There were potentially damaging defeats for Sam Querrey and Pablo Carreno Busta in the second round of the Paris Masters on Tuesday.
Angelique Kerber, who has not won a title this year, made a losing start to her last tournament of 2017, the WTA Elite Trophy.
The one match Rafael Nadal needs to win to claim the year-end number one will come against Chung Hyeon at the Paris Masters.
Gael Monfils' injury woes have dealt the Frenchman another blow, with his year ended on Sunday and causing him to miss the Davis Cup final.
It was not straightforward, but Roger Federer claimed an eighth Swiss Indoors title against Juan Martin del Potro.
Jo-Wilfried Tsonga's eight-match winning run came to an end in the Austrian capital, where Lucas Pouille was crowned champion.
Venus Williams had won her previous seven matches against Caroline Wozniacki, but she was no match for the Dane in Singapore.
Lucas Pouille and Jo-Wilfried Tsonga set up an all-French final in Vienna, with the latter having won their previous two meetings.
Sunday's final of the Swiss Indoors Basel will feature seven-time champion Roger Federer and a rejuvenated Juan Martin del Potro.
After beating Caroline Garcia to stand one win away from WTA Finals glory, Venus Williams said: "This tournament is one of the best."