Novak Djokovic shook off a back problem to beat Britain’s Cameron Norrie in four sets on Friday to became the oldest man to reach the US Open last 16 since Jimmy Connors in 1991. The 38-year-old Djokovic defeated Norrie 6-4, 6-7 (4/7), 6-2, 6-3, staying in the hunt for an unprecedented 25th major title. But the Serbian star admitted whether his body can still withstand the grind of a two-week long Grand Slam is potentially beyond his control.

“In terms of how I’m feeling, it really goes up and down, to be honest,” said Djokovic, who was compromised by injury at the Australian Open and Wimbledon earlier this year. “It’s frustrating for me, honestly, that I’m not able to feel 100 percentage always like I have for 20-plus years.

Britain’s Cameron Norrie during his defeat to Novak Djokovic