Britain's Andy Murray was beaten 6-1 6-3 by Croatian teenager Borna Coric in the Dubai Championships quarter-finals.
Murray, ranked three in the world, was dominated by the world number 84.
Coric, 18, broke the 27-year-old three times in the first set to lead the dual major winner and followed up in the second by breaking in the sixth game.
Murray, a beaten Australian Open finalist earlier in the month, failed to get a break point and made 55 unforced errors in a poor display.
Coric is the first teenager to reach the Dubai semi-finals since Rafael Nadal clinched the title as a 19-year-old in 2006.
"[It's] one of the biggest wins, for sure," said Coric. "I was just trying to maintain my level, stay in the rallies as long as I can, which I was doing really well. I was also running very well.
"I'm still a little bit struggling with the best players when they are playing really good obviously. I always need to be on 100 per cent, and when I'm on 100 per cent, mentally [and] also tennis wise, then I think I can play with them."
Murray paid tribute to his opponent, who is aged 18 years and three months.
"He didn't make many errors," said Murray. "He played very solid and he moved well. I made way too many mistakes from the beginning of the match right through to the end, early in rallies, rushing points."
The Scot thrashed Coric in a Davis Cup singles tie in September 2013 in their only previous meeting, but was second best throughout Thursday's encounter and was beaten in one hour and 19 minutes.
Coric, who won 66 points to his rival's 38, will face Roger Federer in the semi-finals after the Swiss world number two progressed when Richard Gasquet retired injured at 6-1 down in their quarter-final.
The Croatian is the youngest player in the men's top 100 and sealed victory in the quarter-finals when Murray skewed a regulation forehand return wide of the mark.
|Who is Borna Coric?|
|Won 2013 US Open junior title||Beat Rafael Nadal to reach semi-finals of Swiss Open last year|
|Youngest current player in the men's top 100||Says his goal in tennis is "to become number one"|