BEIJING: Novak Djokovic said this year has been a massive struggle, but insisted it was not due to stamina issues and was instead linked to an undisclosed health problem, after requiring another medical timeout at the China Open on Friday.

The 24-time Grand Slam champion battled back from a set down to beat local hope and world number 104 Bu Yunchaokete 4-6 7-6(2) 6-2 in the last 16 in Beijing, extending his perfect record at the tournament to 31-0.

Djokovic, who returned to the ATP 500 event after an 11-year absence as he seeks to regain momentum following his exit from the top 10, is aiming for a strong finish to a season that included a shock first-round loss at the US Open, his earliest Grand Slam defeat since the 2006 Australian Open.