With Muster at his mercy, the tall 19-year-old moved in for a forehand volley which could be described as routine by professional standards, only to belt the ball into the court in front of the net.
"I was not watching the ball very carefully," Schalken admitted. Muster required no further encouragement, recovering to win his 33rd consecutive match on clay, 5-7, 6-1, 6-4, and advance to the semi-finals where he will meet Cedric Pioline, the 1993 runner-up, in defence of the Monte Carlo Open.
A year ago, Schalken showed his potential by winning a Challenger grade event in Monte Carlo, which carried a first prize of $7,200 (pounds 4,800). Yesterday, although disappointed not to have executed better, he left with $48,500.
Had the top-seeded Muster fallen to the world No 65 yesterday and followed the likes of Andre Agassi, Michael Chang, Boris Becker, Goran Ivanisevic, Sergi Bruguera and Jim Courier through the exit, some observers might have concluded that one of the top nine events on the ATP Tour had been reduced to Challenger status.
While that view is understandable, it underestimates the outstanding progress made by a new generation. Schalken was the youngest of the quarter- finalists, and two players aged 20, Marcelo Rios, of Chile, the No 13 seed, and Spain's Alberto Costa, seeded No 16, advanced to meet in today's other semi-final.
Rios, who beat Becker on Thursday, continued to impress yesterday when he defeated Sweden's Magnus Gustafsson, 6-3, 6-4. Costa, having dispatched Agassi, overwhelmed a compatriot, the qualifier Felix Mantilla, 6-3, 6- 3.
Muster being the fierce competitor he is, Schalken's miss may only have denied spectators a little extra excitement. "He definitely had a chance to go up 3-0, even if it was only with one break," the Austrian said. "On the other hand, he played so many lines that sooner or later the luck had to be over. Maybe that was the sign.''
Schalken's prospects diminished faster than they had materialised. Muster broke back to 2-1, held to deuce and did not lose a point in the next three games to be serving for the match at 5-2.
Fortunately for Schalken's peace of mind, the Dutchman was able to summon sufficient resistance to force Muster to serve for the match a second time, which he accomplished without dropping a point to triumph after nearly two hours.
A 90-minute rain delay can be added to that. Spectators waited patiently for the match to resume with Schalken to serve for the opening set, having broken for 6-5.
While the Dutchman had needed no urging to head for the locker-room to rest, Muster almost had to be dragged off the court. The Austrian brandished his racket, demanding to play on, and did not leave until told to do so by the ATP Tour supervisor, Gayle Bradshaw.
"I think it was nonsense to stop the match at that time," Muster said. "If one of the players complains that the court is bad, or slippery or dangerous, I agree, but why has the umpire to get into the match and say it's dangerous? The court was OK. We could have continued playing.''
Although Schalken managed to overcome the weariness in his play during the second set, he discovered to his cost that "it's not easy to kill points with Muster at the other side of the net".
Results, Sporting Digest, page 27