Top seed John Isner suffered a 7-6(3) 7-6(5) defeat at the hands of fellow American Taylor Fritz in the second round of the Auckland Classic on Wednesday.
The big-serving Isner, ranked 10th in the world, fired 26 aces but struggled in his second serve as Fritz took full advantage to edge the match in an hour and 39 minutes.
"I really wanted to get the win on him. I knew I could," said Fritz. "We had a lot of close points and it just feels really good to get through that," said world number 50 Fritz.
The 21-year-old converted one out of three break points and saved three on his serve.
"I like to think that one of my strengths is playing under pressure and playing the big moments well," Fritz added.
"More often than not, the matches I lose, that`s where I get let down. So I`m happy to start the year ... playing well under pressure and handling those big moments well."
Fritz will next face Britain's Cameron Norrie, who overcame Joao Sousa 7-6(5) 6-4.
American Tennys Sandgren also advanced to the quarter-finals, beating third seed Marco Cecchinato of Italy to set up a meeting with Leonardo Mayer.
The latter saved five out of six break points to move past Matteo Berrettini 5-7 7-6(5) 6-3.