“But as it’s not like in 18 hours, like you got a third round to play, semi-finals, you have an extra day, adrenaline, there’s a lot of things. Two good nights of sleep, doctors, physios.
“Hopefully we’ll find out that it’s actually nothing bad, that it was just the groin that went really tight from playing a lot, who knows what, from nerves. I don’t know.
“I’m hopeful. We’ll find out tonight, tomorrow. The next day we’ll see how it goes.”
Federer took the medical timeout during the third set of his battle with Sandgren but he claims the problem arose earlier in the match.
He added: “I felt like it came midway through the second set maybe, after he had broken me at some point. I started to feel like my defense wasn’t really there.