British number one Andy Murray has slipped out of the world's top 10 for the first time since 2008, following Marin Cilic's victory at the US Open.
His ranking of 11 means he faces a battle to secure one of the eight slots available for the end-of-season World Tour Finals in London in November.
Murray has a significant number of points to make up on Tomas Berdych, who occupies the final qualifying position.
The 27-year-old lost to Novak Djokovic in the quarter-finals in New York.
The Scot is due to play three more events before the London tournament and he may need to reach the final of either the Shanghai or Paris Masters to have a decent chance of qualifying.
Since his operation, he had not beaten a top 10 player until overcoming Jo-Wilfried Tsonga in round four at Flushing Meadows.
At the other three majors in 2014, Murray reached the last eight at the Australian Open and Wimbledon, while he was beaten at the semi-final stage of the French Open.