Spain's Rafael Nadal faces the first real test of his title defence when he takes on David Nalbandian of Argentina at the US Open on Sunday.
Nadal won his opening round match against Andrey Golubev in straight sets, then got an early mark in his second round clash when Nicolas Mahut retired at the start of the third set.
But the Spaniard knows Nalbandian is a different proposition altogether. The pair have split their four previous meetings.
"For me, he's a very dangerous opponent. I have had very tough matches against him," Nadal said. "He's a very good player. We know each other really good. We practice a lot together. He's a good friend. This will be a very good test for me."
Britain's Andy Murray is also facing a tough match against Spain's Feliciano Lopez, the 25th seed.
Murray survived a scare in his second round clash with Robin Haase, fighting back from two sets down to win in five, and said he needs to improve to stay in the championship.
"I'm sure in the next match I'll play better, I'll strike the ball more consistently and I'll move better," said the Scot. "That's just what I've got to focus on. Take one match at a time, because if I play like that then I'm not going to win the tournament and I won't give myself much of a chance."
The women begin their fourth round matches on Sunday, playing for spots in the quarter-finals.
Last year's runner-up, Russian Vera Zvonareva, plays Sabine Lisicki in the featured night match while Australia's Sam Stosur tackles Russia's Maria Kirilenko in the day match on the Arthur Ashe Stadium centre court.
"I think the whole draw is very tough and anyone can win," Zvonareva said. "There is such tough competition right now. Anyone out there who is going to be able to produce the best tennis, this week, next week will be winner."