Murray took the world number three ranking for the first time this week
Andy Murray suffered a straight sets 7-6 6-3 quarter-final defeat by Juan Martin del Potro at the Madrid Open.
World number three Murray was 5-2 up in the first set but eventually lost on a tie-break as his Argentine rival upped his game.
Del Potro, ranked number five in the world, had never beaten Murray in three previous meetings but grew in confidence and form in the second set.
Murray failed to deal with his rival's play and Del Potro closed out the win.
Del Potro will now play Roger Federer, who battled past Andy Roddick 7-5 6-7 (5-7) 6-1.
Federer improved his career record against the American to 18-2 but it was not straightforward, with Roddick recovering from 3-1 down to win the second set tie-break before the world number two dominated the decider.
In the other half of the draw, Rafael Nadal saw off Fernando Verdasco.
The world number one will now face Novak Djokovic, who beat Ivan Ljubicic 6-4 6-4 on Friday, in the semi-finals.