World No 1 Novak Djokovic and No 2 Andy Murray crashed out of the Western and Southern Open within an hour of each other, before Rafa Nadal restored order with a win over old rival Roger Federer.
With the sprawling Lindner Family Tennis Centre still buzzing over John Isner's 7-6 (7-5), 3-6, 7-5 quarter-final upset of Djokovic, another bombshell dropped on the nearby grandstand court as Tomas Berdych swept past US Open champion Murray 6-3, 6-4, creating shockwaves that will carry through to Flushing Meadows.
The upset of two of the grand slam's biggest favourites have thrown the tournament wide open and added new contenders to the usual suspects.
But world No3 Nadal underlined his claims by taming Swiss arch-rival and five-times Cincinnati champion Federer 5-7, 6-4, 6-3.
Federer and Nadal, meeting for the 31st time in their storied careers, treated the packed centre court to high-quality tennis that had some convinced that the Swiss maestro, winner of a record 17 grand slams and the muscular Spaniard, 12-time grand slam champion, were still the two top players in the world.
"I'm happy with my progress along the way," said Federer, who will drop to world No7, his lowest ranking in nearly 11 years.
"Could have won tonight, should have won tonight. But at the end, I think Rafa's confidence and the way he's playing at the moment got him through," he said.
In a heavyweight match-up featuring two of the game's big hitters, it was Isner who topped Djokovic after a slugfest lasting two hours and 23 minutes. The towering American will face Argentine Juan Martin Del Potro in the last four.
Djokovic, a four-time runner-up on the Cincinnati hard courts, had been eager to take the final step this year and become the first player to complete what the ATP Tour has dubbed the "Career Golden Masters" by winning all nine World Tour Masters titles.
But US No1 Isner, ranked 22 in the world, has been in good form on hard court surfaces with a win in Atlanta and a runner-up finish in Washington.
"I was putting the ball on the court and I was feeling good off the baseline, I felt like, when I had a chance. I played aggressively," said Isner. "I think I was able to put a little bit of pressure on him."
Murray said he struggled to come to grips with the fast surface but the sixth-seeded Berdych was always going to represent a tricky test for the Wimbledon champion with the Czech holding a 5-4 advantage in head-to-head meetings.
The Scot, a two-time winner in Cincinnati, had just four breakpoint chances against Berdych and failed to convert them all. "You know, the things that I really need to do well - serving, returning, moving - that was good," said Murray. "But I'll need to work on my groundstrokes a bit, make sure I'm not making too many mistakes going into the US Open."
Del Potro, the 2009 US Open champion, continued to sharpen his hard-court game before heading to Flushing Meadows, beating Russian Dmitry Tursunov 6-4, 3-6, 6-1.