Milos Raonic said he has been playing with pain since January, after scraping his way into the US Open third round on Wednesday.
Raonic was plagued by back issues in New York, but he managed to surpass Fernando Verdasco 6-2 6-4 6-7 (5-7) 7-6 (7-1).
The world number 10 will face Verdasco’s fellow Spaniard Feliciano Lopez next – the pair splitting their two matches in 2015 so far.
The last time they met – in the Australian Open fourth round when Raonic won in five sets – the Canadian was playing without any inhibitions.
Since, however, Raonic has been below 100 per cent – retiring during the Monte Carlo Masters in April with a pinched nerve in his right foot which required surgery that ruled him out of the French Open.
“It’s been a while. I’d say probably Australia time,” Raonic told a news conference of the last time he played without some discomfort.
“But it’s just a tennis player’s life. We don’t have substitutes or anything. I think there’s always something nagging at you.
“It’s tiring. Sort of when you wake up every morning you’re sort of asking yourself, ‘Am I going to be better today?’
“Sometimes you get positive signs from your body and sometimes you get disappointing signs.”
Raonic’s start to the North American hard-court swing was also delayed by his foot problem, and he came into the US Open off first-up losses to Ivo Karlovic (Montreal) and Lopez (Cincinnati).
The Canadian said the back injury flared against Karlovic at the Rogers Cup, but declared it unrelated to his foot concern.
Given his lack of match practice, with only two under his belt between Wimbledon and Flushing Meadows, Raonic said the more time on-court he gets, the better he will be.
“That mental fortitude today is what has gotten me through, and it’s what’s going to give me a chance in my next match as well,” the 24-year-old said.
“I just have to keep plugging away and trying to be better and better each match. Those are the goals.”