The world number five booked his place in the last four but had to go the distance to overcome the 21-year-old Bulgarian, who he believes has what it takes to go to the top.
"I was nervous," Nadal told reporters. "I accepted that my opponent was playing well and I tried to find a solution to win.
"Finally, I was lucky. I found a solution at the right time.
"He's a great player and he has all the shots. He has everything to be a really, really good player."
The Spaniard will face Jo-Wilfried Tsonga in the semi-finals and he said that he will have to be at his best if he is to give himself a chance of winning his ninth-successive title in Monte Carlo.
"I need to play well," he added. "When I play against the best players in the world, the only thing I am thinking is try to play my best and hope my opponent doesn't have his best day."