Novak Djokovic suffered a shock defeat to Tommy Haas to leave Andy Murray in the box seat at the Sony Open in Miami.
Haas completely outplayed the world number one for a 6-2 6-4 victory meaning Murray, who earlier beat Andreas Seppi, is the highest seed left in the tournament going into the quarter-finals.
From 2-1 down in the first set, Haas won seven straight games to go 6-2 2-0 up and, though Djokovic eventually broke back to level, the 34-year-old held on for his first victory against a world No 1 since 1999.
Haas held his first two service games to love and then sealed the first break of the match as Djokovic netted, and the Serbian handed him another with a poor shot at net.
The 15th seed took the first set in less than half an hour, then broke to open the second, in part thanks to a double fault from his opponent.
Djokovic saved break points in his next service game too, before holding to stop the rot, then pounced on some rare unforced errors from Haas to seal a break of his own and level at 3-3.
Haas broke again though at 4-4 and it proved decisive as he sealed the match at the second attempt with a forehand winner.
The German's prize is a last-eight meeting with 11th seed Gilles Simon, who beat Janko Tipsarevic in three sets.
Murray breezed into the quarter-finals, where he will face Marin Cilic, after cruising past 16th seed Seppi.
Murray has yet to drop a set in this tournament and this did not seem likely to change today as he was rarely tested by his Italian opponent, whose sole break point opportunity came in the final game.
The Briton felt the windy conditions worked to his advantage.
"It was swirling a little bit at the start and he made more unforced errors than he usually does," he told the Independent. "I was able to track down a lot of his balls because he wasn't timing his shots that well.
"But he started going for his shots more in the second set and made it tough for me."
In the women's draw, Serena Williams matched Steffi Graf's record for the number of match wins in this tournament, a 6-3 7-6 (7/5) win over Australian Open finalist Li Na in the quarter-finals seeing the American claim her 59th victory in the Miami Masters.
"I'm excited," said the five-time champion in her on-court interview. "It's just great.
"Every day I play here is a surprise for me because I enjoy it so much. I enjoy the great crowd and I'm just happy to still be here."