Second seed Rafael Nadal made light work of Frenchman Jo-Wilfried Tsonga to reach the last 16 of the US Open.
Nadal will now face fellow Spaniard David Ferrer
Tsonga, who beat Tim Henman in the second round, put up a stern challenge in the opening set before Nadal clinched it on a tie-break.
Thereafter, the Spanish world number two, who has been suffering with a knee injury, took control.
He broke twice to take the second and won 10 of the last 11 games for a 7-6 (7-3) 6-2 6-1 win.
Nadal once again had strapping on both knees, but he looked much more at ease than in the first two rounds when he struggled with his fitness and needed courtside treatment.
"I feel so much much better today so that's good for me," he said.
Third seed Novak Djokovic demonstrated why he is regarded as one of the title favourites with a 6-1 6-3 6-4 win over Argentine Juan Martin del Potro.
Djokovic was in irresistable form, converting every break point he earned against his 18-year-old opponent.
The Serb faltered once, when serving for the match at 5-3, but he recovered to break serve for the fifth time to seal victory in one hour 42 minutes.
"It was very important to get it done quickly," he said. "That's why I was very concentrated and aggressive from the start.
"Hopefully I will now play even better the next round."
Djokovic will face another Argentine, Juan Monaco, in the last 16.
Monaco beat compatriot Agustin Calleri, who upset Lleyton Hewitt in the second round, 7-6 6-2 6-4.
Fifteenth seed David Ferrer reached the fourth round in New York for the first time but only after saving a match point against David Nalbandian.
Argentine Nalbandian was a break up in the fourth and fifth sets but each time the Spaniard fought back.
Then 24th seed Nalbandian, who reached the semi-finals in 2003, was broken at 5-5 in the deciding set.
Ferrer then served out for a 6-3 3-6 4-6 7-6 (7-5) 7-6 win and a place in the last 16 against fellow Spaniard Nadal.
Juan Iganacio Chela held his nerve to defeat 12th seed Ivan Ljubicic in five sets.
The Argentine 20th seed took the first set but lost the second on a tie-break.
Ljubicic took the third set but Chela broke at 4-3 in the fourth to level the match and then swept to a 5-0 lead in the decider, only to be broken twice.
However, the 28-year-old then broke the Croat for a 5-0 lead in the decider to wrap up a 6-4 6-7 (5-7) 2-6 6-3 6-3 win after nearly four hours on court.
Chela will face Stanislas Wawrinka of Switzerland in the fourth round.
The unseeded Wawrinka reached the last 16 of a Grand Slam for the first time when he came from behind to shock American Robby Ginepri 5-7 6-4 4-6 6-4 6-3.
Spain's Carlos Moya reached the fourth round of the US Open for the first time since 2003 with a hard-fought 4-6 7-5 7-6 4-6 6-4 win over Philipp Kohlschreiber of Germany.
Moya will meet Latvian Ernest Gulbis, who stunned eighth seed Tommy Robredo of Spain 6-1 6-3 6-2.
Robredo managed just nine winners, compared to Gulbis's 39, and also failed to break serve off his three chances, Gulbis breaking six times.