The birthday boy completed the semi-final line-up by overcoming Richard Gasquet 4-6 6-2 7-5 after a ferocious and engaging battle under floodlights.
It was a high-octane encounter between two determined competitors, however the physical nature of the tussle eventually took its toll on Gasquet’s weary limbs in the closing stages.
The Frenchman, who enthralled the crowd with his dropshots and creativity, was forced to accept defeat when his youthful opponent threaded an exquisite backhand winner down the line to bring the match of the tournament to a close.
Gasquet has slipped from view in the last 18 months due to various injury problems, but his one-handed backhand, which inflicted so much damage, remains a sight to behold in full flow.
Zverev has been taken the distance in all three of his matches in the Cote d'Azur – yet he remains alive in the competition. His shotmaking skills have always been known, but he has the heart to match his talent.
Whether the German has anything left in the tank for Saturday afternoon’s clash with Kei Nishikori remains to be seen, but at least he can enjoy a well-earned slice of birthday cake before contemplating his next challenge.
“The third set was very intense, a lot of tight moments. I feel like in the tightest moments we both played the best tennis,” said Zverev. “I'm just happy with my performance tennis-wise, also mentally-wise.”
Nishikori was also engaged in a protracted affair. The Japanese remained on track for a third clay-court title by defeating Marin Cilic 6-4 6-7(1) 6-3 and reaching the last four.
The world No.36 has endured a tough time with injuries recently, but he is now just a victory away from reaching a first Monte-Carlo final.