"I built confidence for myself, knowing that I can resist almost four hours on court at a good intensity. I'm very happy for that," Nadal said. "It's always better winning in two hours than in four. It was a good test for me. It was a lot of hours on court, moments under pressure.
"There were 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.
"In the third and fourth my serve started working better, and probably that's why I am here with a victory."
The Spanish top seed will next test himself against 2017 Wimbledon finalist Marin Cilic, who won his 100th match at a Grand Slam against 10th seed Pablo Carreno Busta, 6-7 (2-7), 6-3, 7-6 (7-0), 7-6 (7-3) with 20 aces over nearly three and a half hours.
"It was up and down a lot, a difficult match," the Croatian said. "Conditions were a little bit more humid than before. Pablo was playing really good, very solid, and served really good today."
Women's second seed Caroline Wozniacki reached the last eight for the first time in six years with a 6-3, 6-0 defeat of Magdalena Rybarikova.
Wozniacki takes a 4-2 WTA-level record into her next match with Carla Suarez Navarro. Suarez Navarro turned in a heroic effort on Sunday, fighting back from a set and 4-1 down to overhaul Estonian Anett Kontaveit, 4-6, 6-4, 8-6.
Spain's world No. 39 swept the last five games of the second set to pull herself back into the fight, advancing to her sixth Grand Slam quarterfinal and third in Melbourne.
"I've played Caroline a lot of times," Suarez Navarro said. "It will be a really tough match. She's fighting every ball, but at the same time she can play aggressive. She has a lot of experience in these rounds."
Wozniacki took just 63 minutes to advance over Rybarikova, finishing with 25 winners as she won 11 of the last 12 games.
"I felt good about my game," Wozniacki said. "I'm in a good place, I'm hitting the ball well. Everything has been going well the last year and a half or so. It's been good."
Belgian Els Mertens, who missed a chance to play qualifying a year ago, finds herself in the last eight through a defeat of Croatian Petra Martic, 7-6 (7-5), 7-5.
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