The Spaniard, seeded sixth, changed pace early in the second set of his quarter-final tie with the Dutchman to win 1-6, 6-2, 6-4 and qualify for his second consecutive semi-final on clay after Barcelona last week.
Moya will be one of two Spaniards in the last four. Alex Corretja, seeded 12th, humbled the Norwegian qualifier, Christian Ruud, 6-2, 6-0.
Moya said: "I slowed my serve a little bit at the start of the second set, because trying to hit too hard in the first set I realised I was giving him chances to go to the net. It's useless on clay. You just need a different mentality."
Moya, the sixth seed, has achieved his best results on hard courts despite being bred on slow clay like most Spaniards.
Krajicek is not a clay-court specialist, despite reaching the French Open semi-finals in 1993, and it showed yesterday as he relied too heavily on his serve and volley game and was far too impatient.
"I'm not afraid of anybody on clay," he said. "But physically I was not in good shape. I felt a little bit sleepy. I was just tired and I was rushing too much to finish points. It's just tough to play on clay. It takes a little bit of time to move the right way."
Not in Monte Carlo, but on his way to recovery is Tim Henman. The British No 1 is back in training after a lengthy injury lay-off and looking forward to Wimbledon. Henman underwent an operation on his elbow shortly before Britain's Davis Cup defeat by Zimbabwe earlier this month and has been forced to miss several tournaments.
Now Henman, ranked No 15 in the world, is looking forward with confidence to his comeback, starting at the Italian Open on 12 May. That event will form part of his preparations for the French Open along with an event at St Poulton in Austria, before he moves on to Wimbledon.
Yesterday Henman was keen to get back on court and test the elbow under match conditions. He said: "I am happy to say that I am back practising and have been since Monday without any pain."Reuse content