A week after the start of the tournament, when most of the top players breezed through their matches without too many problems, the US Open suddenly gets serious on Monday when the seeds start to square off against each other.
Five-time US Open champion Roger Federer faces his first real test after cruising through the opening week when he takes on Austrian Jurgen Melzer in the fourth round while world No3 Novak Djokovic tackles in-form American Mardy Fish.
Things will also get a lot tougher for the leading women with Danish top seed Caroline Wozniacki meeting Russia's former champion Maria Sharapova in a match worthy of a final.
Wozniacki, a finalist in New York last year, will go into the match as a slight favourite after she won three lead-up tournaments last month and conceding just three games in her first three matches.
"She has won grand slams, I haven't," Wozniacki said. "But I don't feel the pressure. You know, my time will come. If it won't come now, it will come tomorrow, so I'm not afraid of that."
Sharapova, slowly regaining her fitness and confidence after struggling with injuries for two years, said her past experiences of winning grand slam titles would count for little, against the in-form Wozniacki.
"She was the finalist here last year, so she's accustomed to the court and the atmosphere," said Sharapova, seeded 14th this year. "She obviously knows how to play good tennis on the court. It will be interesting."
Federer won his first three matches without dropping a set and is displaying the sort of form that has enabled him to win 16 grand slam titles. He is an overwhelming favourite to beat Melzer after he won their only previous meeting in straight sets.
"Melzer is a good friend of mine. I played him in Wimbledon, so we'll see," Federer said.
After a slow start to the championship, Djokovic is approaching the level of play that saw him make the US Open final in 2007. But he remains wary of Fish, the only American left in the bottom half of the men's draw.
"He's playing maybe his best tennis at this moment. He's moving really well," Djokovic said.
"He's serving as good as he served always. He has a lot of talent. He's recognizing the moment, coming to the net.
"He has a lot of variety in the game. I have to be on the top of my game to be able to win."
