Nadal, Thiem and Wawrinka are yet to drop a set this tournament while Murray has lost three, and their build-ups were rather different too.
Murray, who lost back-to-back matches to Borna Coric and Fabio Fognini in Madrid and Rome, said: "They are all obviously playing extremely well.
"Rafa's had a great clay-court season, as has Thiem. Stan this tournament has played great. He won in Geneva so is obviously confident.
"I came in playing garbage. I'm the odd one out in the semis, but hopefully I can keep it up."
Murray's match against Nishikori was not the best advertisement for his improved form.
Both men struggled with the cool, windy conditions, and after a terrific first set Nishikori's play was up and down.
Murray was just the steadier, although he also played some poor games, dropping serve twice in the third set - both times immediately after breaking Nishikori - and once to start the fourth.
The good news was that he found a way to get over the line reasonably comfortably and will now hope to replicate last year's win over Wawrinka at the same stage, one of his best ever performances on clay.
Murray said: "It wasn't the best match. It was tricky out there.
"Obviously if someone had offered me a semi-final spot before the tournament, I would have signed up for that because I was not playing well at all. And practice also was not good.
"It's been really good so far. I want to keep going."
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