Karatsev, the Russian ranked 121 in the world, was attempting to reach the last four having had to go through qualifying.

Zhang, ranked 99 and the first Chinese player to make an ATP 1000 event last eight, was attempting to break further new ground by reaching the last four.

"I am happy with my condition. Back to the top level," Karatsev said. "Playing well and feeling well."

Between them the two men had knocked out six seeds to reach the last eight with Karatsev dropping only one set in seven matches, as he eliminated second seed and compatriot Daniil Medvedev in the round of 16.

On Thursday, Karatsev kept his nerve better at key moments and saved all three break points he faced in the first set before winning the tie break. He immediately broke in the opening game of the second set before going on to win in one hour and 40 minutes.

Karatsev reached the Australian Open last four in 2021 but had never reached the last four in a Masters 1000 event.

He climbed to 14th in the rankings in early 2022 before sliding. The 29-year-old is now 121st and had to go through qualifying to make the main draw in Madrid.

"I started the year inside the Top 100 then I dropped and lost some matches. You have to keep going and believe," Karatsev said.

"From the qualifying, match by match, it has got harder, so mentally you have to be there more. Because your opponent doesn't give you any free points so you have to be there yourself. The important thing is now to recover well."

In the semi-finals, Karastev will face either world No. 5 Stefanos Tsitsipas or 65th-ranked German Jan-Lennard Struff.