Rafael Nadal has suggested he could walk away from tennis if he feels he can no longer play at a "high level", although the world No 2 remains confident he can win more titles.
Nadal has endured a turbulent few weeks after slipping to surprise defeats in Monte Carlo and Barcelona last month, before being hit by a stomach bug on Sunday.
Stefanos Tsitsipas is coming off a three sets win over Rafael Nadal that took 27 games to decide. But Djokovic has a peerless ability to smother those fizzing bombs as they rear up off the surface - a skill honed during his seven wins over Nadal on clay. But this year it hasn't been the case.
"After Indian Wells, I had a big mental drop", Nadal said.
Second-ranked Nadal had beaten Tsitsipas in all of their three previous encounters, but the 20-year-old Greek played more aggressively this time.
"I'm really happy that I managed to keep my nerves down and fight back. I couldn't do much", Tsitsipas said. "Adding variety and being unpredictable was the key today".
"I really liked my fighting spirit", he said.
"He played quite smart, moving me around the court".
The 18-year-old Auger-Aliassime is having his breakthrough season on tour, having moved to 30th in the rankings after reaching the final in Rio and the semifinals in Miami.
Although Nadal might be in poor form, the author feels he could still win against the best players, even on his worst day.
The world No. 2 will be defending his title in Rome after beating Germany's Alexander Zverev a year ago and has been given a bye into the second round, where he will face one of two Frenchmen, Richard Gasquet or Jeremy Chardy.
Tennis TV described the moment as "scary good" on social media - the the match commentators were hard pressed to find a better adjective.
A third Madrid Open title means that Djokovic now has 33 Masters 1000 wins.
Nadal, meanwhile, was left reflecting on a third straight semi-final exit on clay after losing to Fabio Fognini in the Monte Carlo Masters and Dominic Thiem in the Barcelona Open. "During all my life, I think I have taken the victories very naturally and with a lot of normality".
"If I'm not able to do these things (play) properly, that is the end of the situation, period".
This was a straight-sets win, but still a draining one, as Djokovic needed 2hr 22min to wrap up a 7-6, 7-6 victory.
"It's more normal what is happening right now than what happened in the last 14 years", he said.