World number one Rafa Nadal fell to Cyprus's Marco Baghdatis 6-4 4-6 6-4 in the quarter-finals of the Cincinnati Masters on Friday as three of the top four seeds lost out.
Britain's Andy Murray and Serbian Novak Djokovic both suffered defeats to American opponents with Mardy Fish and Andy Roddick advancing to meet each other Saturday.
The only member of the top four to progress was third-seed Roger Federer, who meets a confident Baghdatis for a spot in Sunday's final after defeating Russian Nikolay Davydenko 6-4 7-5.
Nadal was well below his best against an inspired Baghdatis, whose big serve was on full power - he blasted 18 aces while the Spaniard could manage just four.
The match turned decisively in favour of the Cypriot, who had lost all six previous meetings with Nadal when the Spaniard double-faulted at break point with the game at 4-4 in the third.
It was a mistake the Nadal described as "criminal" and the 20th ranked Baghdatis served out for the win, giving himself a major confidence boost ahead of the US Open, which begins on August 30.
"I'm playing good; I'm feeling happy on court. I'm out of injuries. It's nice when you wake up and you don't have pain somewhere," said the Cypriot, beaten finalist in Washington earlier this month.
An All-American semi-final is a timely boost for the game in the US which is currently without a top ten male player -- although Roddick's win assures he is back to at least ninth in next week's rankings.
Roddick and Fish are close friends who, since their high school days, have spent many hours together practicing. "There won't be many secrets that's for sure," Roddick said after beating third-ranked Serb Djokovic 6-4 7-5, his booming serve looking back to its best.
"We know each other probably better than any other two guys on the tour," added Roddick, who has been struggling for fitness and recently revealed he has been hampered by mononucleosis.
But, after his win over fifth-ranked Swede Robin Soderling on Thursday and his confident display against Djokovic, the Texan is looking in good shape for the U.S Open.
"Once you decide you are playing you forfeit any excuses," Roddick said when asked about his health.
Fish took advantage of an exhausted world number four Murray, who wilted in 100-degree heat (38 Celsius) and lost 6-7 6-1 7-6 to the unseeded American.
Fish, ranked 36th in the world, has now beaten the Briton in three successive meetings following victories in Miami and at Queen's Club, London.
Fourth seed Murray won the opening set tiebreak despite producing his first double-fault at 5-5 but the tired Scot then struggled in a one-sided second set.
Both players held serve throughout the decider and Fish emerged victorious against a frustrated Murray who, after losing a rally on the penultimate point, thrashed the ball out of the stadium.
Federer, who had a walkover in the third-round after having had his opponent retire in the first set of his second-round match, was pleased to have quickly adjusted to the slick surface at Cincinnati and book his place in the last four.
"I wondered if I had enough to play on these kind of courts in a match moment," Federer told reporters. "I was really hoping to get a decent match in today, play solid, and come through and then feel like I am really in the tournament.
"Because before it didn't really feel that way. So there's a sense of relief from my side. Right off the bat kind of beating a top 10 player is not so easy, you know."