Nadal has had a number of knee problems over his career but this latest issue emerged towards the end of 2017 with the Spaniard pulling out of November’s ATP World Tour Finals after just one match.
The 31-year-old then had to pull out of the Brisbane International, his traditional Australian Open warm-up tournament, but has no signs of knee trouble so far this week.
And after coming through a 237-minute fourth-round clash with Schwartzman, Nadal insisted he was glad to have been pushed harder after three straight-sets victories.
“Of course, [I’m] a little bit tired,” Nadal said.
“But I was able to keep fighting until the end.
“It’s the first big match I’ve played in 2018.
“I started later than usual so one match like this probably helps.
“It’s confidence for myself, it’s confidence knowing I can resist almost four hours on court playing at a good intensity.”
He added: “It was a good test for me. It was a lot of hours on court. Moments under pressure.
“So, yeah, a lot of positive things that I managed well. After the second set, it was a very tough moment.
“The third set was a great set to win, being honest.
“My serve start to work much better after the second.
“I started to win some free points again with the serve, because for a set and a half almost I didn’t win a free point with my serve.
“So that’s not good news during that part of the match.
“That’s why I lost that many chances, three times the opportunity to be break, to confirm the break, and I didn’t.
“Then in the third and fourth my serve started working better, and probably that’s why I am here with a victory.”