Defending champion Andy Roddick maintained his blistering form at the US Open with a 6-1 6-3 6-3 win over Argentina's Guillermo Canas.
The second seed faced two break points in the opening game of the match, but having saved those he was never in trouble again.
Canas won a total of just 17 points on the American's serve and was broken five times.
Roddick goes on to face Tommy Robredo for a place in the quarter-finals.
The American refused to get carried away by his stunning performance against Canas, in which he served 21 aces, no double faults and saved all five break points he faced.
"I had a game plan, I executed it. I took it to him," said the 22-year-old, who is yet to drop a set and has lost his serve just once.
"I really don't have a magical explanation for it."
Lleyton Hewitt, another contender for the title, reached the fourth round with a 6-1 6-4 6-2 win over Feliciano Lopez.
The fourth seed dazzled the New York crowd by winning the first 12 points and wrapping up the first set without losing a point on serve.
Lopez dug deep in the second set, but failed to convert four break points while Hewitt only needed one opportunity before taking a commanding lead.
And the Australian broke twice in the third to seal a comfortable win and a meeting with Slovakian Karol Beck, who beat Russian Nikolay Davydenko in five sets.
Tommy Haas took his US Open record to 16-7 with a routine 6-2 6-3 7-5 win over Ricardo Mello in the third round.
The 26-year-old, in the last 16 for the fourth time in his career, will face Tomas Berdych, who outlasted Mikhail Youzhny 2-6 6-1 6-3 4-6 6-1.
Berdych created one of the biggest upsets of the season at the Olympics in August when he stunned world number one Roger Federer.
Talented Swede Joachim Johansson was an early winner on day seven of the tournament, beating Austria's Stefan Koubek 6-7 (2-7) 7-6 (7-1) 6-1 6-3.
Koubek, one of three Austrians in the last 32, defeated seventh seed Juan Carlos Ferrero in the previous round but ran out of steam against his big-serving opponent.
Koubek was followed out of the tournament by his two compatriots.
France's Michael Llodra defeated Jurgen Melzer 6-3 6-2 7-5, while Robredo ended Alexander Peya's hopes 6-3 6-3 6-2.