Top seed Roger Federer was beaten 6-4, 7-6 by American James Blake in the quarter-finals of the Olympic men's singles on Thursday.
The Swiss produced an error-strewn display under the Centre Court lights after a long rain delay and was outplayed as Blake claimed his first victory against Federer in nine meetings.
Defeat continued Federer's miserable season and leaves the 27-year-old running out of chances to win an Olympic medal, although he is still in the doubles here.
Last month he lost his Wimbledon title to Rafael Nadal and he will cede his world number one ranking to the Spaniard on Monday.
Blake said he always believed he could break his duck against the 12-times grand slam champion.
"I always believed in myself," Blake, who is now just one victory away from a guaranteed medal, told reporters. "I'd lost to him eight, nine, 10, 50 times, I don't know how many, but I had the feeling it could be my day today."
Eighth seed Blake played two superb baseline rallies to close out the first set as Federer served at 5-4 down.
Federer then fell 3-0 down in the second set before battling back to 3-3.
Luck looked to be on Federer's side when a nervy volley dribbled over off the net tape when Blake was two points away from victory in the 12th game of the second set.
However, the inspired American would not be denied, surging 6-2 ahead in the tiebreak and sealing victory on his first match point.
"He didn't play a good first game and that put me up straight away. I played loose and free and I always fancy my chances on the 5-4 game," Blake said.
"When he got that net cord I started wondering if it would turn around and he would come back playing like the best player in the world. But I know I can beat the best in the world on my day and today I proved it."
Blake will play either Chile's Fernando Gonzalez or Frenchman Paul-Henri Mathieu for a place in the final.
