"I don't understand how I can lose a match where I get broken one time in three sets and not managed to get any chances on his serve", said Zverev, who is tipped to one day make world number one.
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. "I would like to play and be 100 percent".
Federer powered past Frenchman Richard Gasquet 7-5 6-4.
The next couple of games went past within a blink of an eye, and Harrison soon found himself serving for the set at 5-3 up.
6-4 in two hours and 35 minutes.More news: Amazon Debuts (Parent-Controlled) Shopping Accounts for Teens
"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. Nadal ultimately prevailed, 6-4, 6-7 (4), 6-3.
Nadal, the top seed, has now booked a semi-final date tomorrow with Croatia's fourth seed Marin Čilić.
The 16-times grand slam victor, who beat Dimitrov in three sets in the China Open semi-finals last week, fired 32 winners and won two crucial breaks to prevail in two and a half hours at the Qi Zhong Centre.
In the third set, Nadal saved the one break point he faced in the match in the fifth game, and broke serve in the sixth to set up victory.
"I think (it was) a great battle between two players who are playing all the time very focused, very intense points".
World number five Cilic secured his place in the semi-finals with a routine victory over Spain's Albert Ramos-Vinolas.