Del Potro's wrist problems have been well documented down the years and at 2-2 in the deciding set he fell awkwardly on to his non-playing hand, dropping on to it with most of his weight.
Roger Federer, the only man who can deny Nadal the year-end world number one ranking, powered past Frenchman Richard Gasquet 7-5 6-4.
"I'm glad to reach the semi-finals in this tournament, but to be honest now, as I say always, health is the first of all of my life", Del Potro said.
"I felt good in the first match", the 36-year-old said.
"But if nothing risky happened, I will be here to play tomorrow and doing my best".
Gutsy Del Potro recovered from a set down to win 3-6, 7-6 (7/5), 6-4 against rising star Zverev and will play Serbia's Viktor Troicki, after he beat John Isner in the final match of the day.
"It was a very tough match".
He stormed back to take the second on a tie-break and Croft says it was down to a change of approach in the Bulgarian's game - and he mentally got the better of the Spaniard.
World No.1 Nadal downed Italian Fabio Fognini 6-3 6-1 while second ranked Federer downed Alexandr Dolgopolov 6-4 6-2.
"Just focus on holding that momentum".
Dimitrov sensed another opportunity in the fifth game of the third set, but Nadal survived the break point.
The Spaniard's victory march was only momentarily held up when a disturbance broke out in the crowd and the players were forced to wait as a small group of spectators was escorted out of the arena.
Dimitrov, chasing a spot at the season-ending ATP Finals in London, beat American Sam Querrey to set up a Chinese reunion with Nadal.
He needed several minutes of medical attention and sat on a chair on the back of the court, wincing in obvious distress.