The world number two took just 61 minutes to reach the last 16 at Indian Wells, but said that he "tweaked" his back in the later stages of the match.
However, he admits it is not an uncommon complaint for him and revealed that he will rest up ahead of his next match.
"At the end tweaked it a little bit," the Swiss told reporters. "But it's not the first time it's happened in my career.
"I know how to deal with it. I'm walking fine. I have a day off tomorrow. Everything is all right."