Zverev's consistency on the ATP Tour allowed him to build up a big lead over Alcaraz as he claimed the No 2 ranking, but that is under serious threat after his latest setback.
After a worrying slide in form over the last few months that included some disappointing courts on South American clay courts before he failed to shine at the Indian Wells and Miami Masters events, Zverev was hoping to get back on track at the Monte Carlo Masters.
Yet he lost a gruelling battle against Italy's Matteo Berrettini, with the former Wimbledon finalists serving up a 2-6, 6-3, 7-5.
"The game plan was the same but I changed my attitude and the way I was believing in my strokes," said Berrettini.
"I wasn't hitting my forehand and serve like I had in previous days and I had to adapt and adjust to the conditions and Sascha was playing unbelievable, so it was not easy. Then I told myself to be more aggressive and if I am going to lose this match, I am going to do the right things and luckily it worked.
"I grew up on clay, playing on it until 19. On Tour, we don't play on it as much but at the same time I have missed the biggest tournament on clay for the past three years and that was tough and now I want to enjoy it. I feel really comfortable on clay."
Berrettini's joy was contrasted by Zverev's despair and he door is now open for world No 3 Alcaraz to overtake the German in the rankings if he wins the title in Monte Carlo.
Even if Alcaraz comes up short in that mission, he has a big opportunity to get closer to the No 2 spot and with Zverev defending 1,000 ranking points from his win at the ATP Masters 1000 tournament in Rome next month, he could get set for a big drop in his ranking unless he finds his form quickly.
"It was in the beginning. Now it's less, because I'm just playing terrible," said Zverev last month, when he was asked about his hopes of overtaking world No 1 Jannik Sinner.
"So for me, I have to find my game before thinking about that, because to become World No. 1, you have to win tournaments. I'm not past getting first, second round at the moment. So I need to figure that out first.
"I think I'm serving quite badly the last few weeks. My second serve is fine, but my first serve is terrible.
"From the back of the court I'm not playing great. It's everything. You can't really name one thing."
Alcaraz's move to the world No 2 ranking ahead of the French Open will be crucial, as it would ensure he would not have to face Sinner until the final.