Gilles Simon has warned that Carlos Alcaraz dominating men's tennis is no certainty.

Alcaraz has enjoyed a stunning rise to the top of the rankings over the past two seasons, with the 20-year-old winning two grand slam titles and 12 trophies overall.

The Spaniard has not won a title since triumphing at Wimbledon in July, however, and has reached only one final since then, losing to Novak Djokovic at the Western & Southern Open in August.

A shock defeat to Roman Safiullin at the Paris Masters this week has damaged Alcaraz's chances of pipping Djokovic to the year-end world number one, and Simon does not feel he is a shoo-in to be the standout player of the next generation.

"There are a lot of people who say: 'He's going to win 20 Grand Slams'," Simon told Stats Perform.

"It's long a career. People say he has got the freeway ahead of him. Yes, but we do not know what's behind him? 

"When [Roger Federer first arrived, we thought he was going to win a lot, and he did. Breaking [Pete] Sampras' record, we were sure of it.

"But the danger came from behind. First Rafa [Rafael Nadal] then Novak, then Andy [Murray]. And I think it will be the same for Carlos. The danger will come from behind."

Simon, who won 14 career titles and reached a high of number six in the ATP rankings, pointed to the likes of world number four Jannik Sinner and number three Daniil Medvedev, though the latter is seven years Alcaraz's senior.

"There are players aged 16, 17, who are playing extremely well and who may also have a similar career path. In the end, he still has Novak and hyper-stable, hyper-strong players like Daniil on hard court or other players on clay who can really cause him problems," Simon continued.

"Sinner and so on, but he may also have two or three guys behind him who, in two or three years' time, we don't know who they are yet, but they'll be up there like him, and we'll be saying to ourselves: 'Ah well, he never had that period when he was supposedly going to win everything with no competition in front of him'.

"There is never no competition. The other players, even if they are not the calibre of Federer or Nadal, they are very strong. Daniil is very strong, very stable on hard court, so he can beat [Alcaraz].

"Carlos seems to have the upper hand. But no, Daniil finds a solution, comes up with something else and beats him. And that is why it's never a foregone conclusion, and why it's so interesting to follow. Otherwise, we would not even be watching the match."

The Rolex Paris Masters became the first Masters 1000 to broadcast its qualifying matches on Twitch, live on Rivenzi's channel.