Dominic Thiem had to back up to go forward.

After repeatedly being overwhelmed by towering Kevin Anderson’s booming serve, Thiem learned his lesson. The young Austrian used savvy court positioning and one of the best performances of his life to earn a 7-5, 6-2, 7-6 (2) rout Sunday and his first trip to the U.S. Open quarterfinals.

“One of my best matches ever,” Thiem had said in an on-court interview. “It’s a dream coming true.”

The ninth-seeded Thiem had lost all six of his hardcourt meetings against the 6-foot-8 Anderson, who came in favored and seeded fifth. But enjoying more space on the bigger Louis Armstrong Stadium court, Thiem backed up as far as he could beyond the baseline, and took the sting off the big South African’s serve.

“First of all, I served really, really well. Not the best percentage, but I almost made every point in the first serve game. So I didn’t face one break point, and I didn’t feel so much pressure on service games,” said Thiem.

“Also, the court made a big difference. It’s a huge court. I could go very far back like I do on clay usually. So there were some good advantages for me. … It’s a huge difference. I played him three years ago on Court 17, which didn’t allow me to go that far back. It helped me a lot. I also did the same in Madrid. It worked out, so I thought why not on hard court? It worked out.”

Thiem, 24, played it safe from the baseline, but pulled off a near-flawless performance. He hit 42 winners to just 13 errors, and served well, dropping just four points on his first serve to reach his first Grand Slam quarterfinal other than a trip at the French Open.

After upsetting last year’s finalist, now he’ll face defending champ Rafael Nadal. He’s just 3-7 against the Spaniard, all on clay.

“It was three very nice experiences and six horrible experiences,” Thiem said. “It’s going to be the first time on hard court which is a completely new experience. I’m really looking forward to play him on hard court for the first time. On clay, it’s one of the biggest challenges in sports to beat this guy or to compete with this guy.

“[When I’ve won] I was really aggressive. I have very powerful ground strokes, and I can even hurt him with them. But the key is to play really fast and powerful, but at the same time not make too many mistakes. I did it sometimes against him, but it’s a risky game style.”

Thiem may need to risk to win.