The top two men in the world were given workouts in their most recent matches, and only one emerged victorious.
Andy Murray, the second seed at the US Open, was beaten all too easily by the spectacular hitting of Croatian Marian Cilic, who reached the quarter finals of a Grand Slam for the first time in his career.
The Scot looked sluggish as he failed to cope with the 16th seed's power, and will not be staying at number two in the world for long with performances like that.
Having reached the final last year, finishing runner up to Roger Federer, Murray was a favourite for the title.
But after losing many ranking points for falling so early this year, he will have to work very hard to regain his status as a world beater.
Federer, however, withstood a mighty comeback by Robin Soderling to reach his 22nd consecutive Grand Slam semi-final.
He should be concerned at how he let the Swede back into the match, and will have to work much harder to shut out a resurgent Novak Djokovic in the semi finals.
Caroline Wozniacki, now the second-highest ranked player left in the draw, will be favourite to make her first Grand Slam final now, having eased past the darling of New York, Melanie Oudin.
The American teenager can be proud of her performance here, proving that her Wimbledon run in the summer was not a fluke.
She really needs to back up this performance with a good showing at a Premier event, but there's no doubt her confidence will now be through the roof.
Dane Wozniacki, on the other hand, should beat world number 50 Yanina Wickmayer of Belgium in the semi.
It's very difficult not to see the winner of the women's tournament coming from the lower semi final.
Can crowd favourite Kim Clijsters overcome the might of Serena Williams? [GALLO/GETTY]
Kim Clijsters, in only her third tournament back, takes on defending champion Serena Williams.
These stars, who are both former world number ones, have so much more experience at this level than either Wozniacki or Wickmayer, and whoever wins will be a sure bet for the title.
The remaining section of the men's draw, which houses both Rafael Nadal and Juan Martin del Potro, is going to prove to be highly entertaining.
The third and sixth seeds respectively, they should meet in the semi final, but with the fiery Fernando Gonzalez and Murray's conqueror Cilic lying in wait, they won't find their paths through a walk in the park.
Of the ten top-five seeds that started the tournament, only four remain after a series of upsets not seen in a Grand Slam for many a year.
But it is Murray who will feel the shock most, and a strong end to the year is desperately needed to restore faith in his unquestionable talent.