PARIS — Sunisa Lee is the reigning Olympic champion and currently one of the top three gymnasts in the world.

Yet the gap between her and No. 1 — fellow American Simone Biles — is so great it almost defies Lee’s comprehension.

“She will go into a pass and I’ll just be like, 'How the heck does she do that?'” said Lee. “And [US Coach] Cecile [Landi] will say, ‘I don’t know.’”

A lot of people don’t know, or even understand. That includes many Americans who tune into gymnastics once during the Olympics, such as for Thursday’s all-around competition where Biles is the significant favorite, in part, because of her ability to raise the difficulty of her routines.

“I don’t know if we will ever see her degree of difficulty in any gymnast ever again,” said Samantha Peszek, a 2008 silver medalist who now works as the gymnastics analyst on NBC.

It has caused Peszek to tailor her broadcasting style to attempt to explain to fans who were first exposed to the sport via the old 10-point system that favored flawless execution over athletic daring.

“My job is, how can I make this sport more digestible for fans who only watch every four years,” Peszek said. “And a lot of fans hang onto that ‘Perfect 10.’”

The current system of scoring in gymnastics is a combination of difficulty — the “D-score” — and execution — the “E-score.” Separate judging panels tally each one. While execution is scored on a scale of 10, and thus a perfect 10 is possible, it doesn’t occur.

The highest execution score in all-around qualifying Sunday was a 9.400 on vault by both Biles and Brazil’s Rebeca Andrade, likely the second-best gymnast in the world. Just breaking an 8.0 on bars, beam or floor is considered elite.

The other tally is difficulty, which follows a scoring system based on what maneuvers a gymnast attempts. This is Biles' secret weapon.

“Simone has taken the sport to levels of difficulty that were almost unimaginable,” said Jordan Weiber, a 2012 Olympic gold medalist and now head coach at the University of Arkansas.

Consider the vault, where Biles 6.400 degree of difficulty isn’t comparable to even her closest challengers, Andrade (5.6) or Lee (5.0). As such, even if all three perform their routine with the same level of execution, Biles would walk out of the rotation leading Andrade by 0.8 and Lee by 1.4.

Extrapolated over all four disciplines and Biles has a massive advantage.

Her total degree of difficulty in qualifying was 25.8 — 6.4 on vault, 6.2 on bars, 6.4 on beam and 6.8 on floor.

Andrade’s total was 23.7 or 2.1 points (8.1 percent) behind Biles. Lee’s cumulative difficulty was 22.8 or 3.0 points (11.7 percent) behind.

By the time you get to the fifth place qualifier — Italy’s Manila Esposito — the gap is 3.6, which means Esposito would have to execute more than 14-percent better than Biles to defeat her.

That’s a steep hill to climb and unlikely without a Biles meltdown. That is what Biles has done by pushing and pushing the envelope concerning what is possible. She is almost playing a different sport. The competition is simply about her doing her job. She doesn't have to be perfect.

How did Biles get to this level? Talent, of course. Hard work, of course. She’s also benefited from the scoring system that favors more experienced athletes who are able to add difficulty.

At 27, she is the oldest American female Olympic gymnast since the 1950s. Because she has avoided major injury while maintaining intense competitiveness and motivation, she has had the time and training to build up on her skills and thus scores.

“The older you get the easier it becomes,” said Alicia Sacramone Quinn, a 2008 Olympic silver medalist and Team USA’s strategic lead. “You are almost on autopilot. You know your body and to know today is going to be a good day to do vault because, ‘I feel like my body can handle that.’”

With USA Gymnastics moving away from the centralized training system employed by former director Martha Karolyi, Biles and others have been able to train how they think is best for their individual development.

“The older you get the more in-tune you get with that,” Sacramone Quinn said. “And I think the fact her gymnastics just keeps getting better the older she gets is a testament to her drive of wanting to be the best and to influence this sport and keep pushing the boundaries to where it is limitless.

“I say to her, ‘Stop being so good at gymnastics,’” Sacramone Quinn joked. “Let everyone else catch up.”

On Thursday they will have that opportunity. Considering the routines Biles is expected to attempt and the difficulty scores she will rack up, whether anyone can catch up remains unlikely.