World number one Novak Djokovic will miss next week's Shanghai Masters after failing to overcome a back problem.
The Serb, 24, aggravated the injury during the Davis Cup semi-final tie against Argentina and was unable to defend his China Open title this week.
He said: "I did another check this week and the results are not good so I need to continue my rehabilitation."
Roger Federer has also pulled out of the Shanghai tournament to "rest and recuperate" after a busy summer.
Djokovic, who first suffered the injury on his way to winning the US Open, has a partial rupture of a back muscle.
The injury forced him to quit his Davis Cup semi-final singles match against Argentina's Juan Martin del Potro last month.
He missed the opening singles matches for the Davis Cup holders with back pain but was named to play in the reverse singles.
He pulled out while trailing 6-7 0-3 and Argentina won the tie 3-1.