As the football world prepares to head to Qatar in six months' time, this World Cup is set to bookend the era in which Lionel Messi and Cristiano Ronaldo have been the sport's two pre-eminent players.

At the time, it felt like the 2018 tournament in Russia marked a turning point as a teenage Kylian Mbappe became a global superstar by helping France become world champions.

Mbappe consoling Messi after starring as France beat Argentina in the last 16 was an iconic image of that World Cup.

Fast forward to the present and Mbappe and Messi are team-mates at Paris Saint-Germain.

Still just 23, the future certainly belongs to Mbappe, who has eclipsed the Argentine at his club this season and has PSG, Real Madrid and football fans the world over on tenterhooks awaiting an announcement on where he will play next season.

Meanwhile, even if they are now on the wane, Messi and Ronaldo will go to Qatar hoping to seize surely their last chance to lift a World Cup, the one glaring omission from the CV of each.

These are the players who have between them won 12 of the last 13 editions of the Ballon d'Or –- Messi won his seventh last year.

Both have won a continental title with their national team, but neither has quite lit up a World Cup in the way they would have hoped.

This will be Messi's fifth World Cup. He was 18 when he scored on his tournament debut in 2006. He inspired Argentina to the 2014 final, which they lost to Germany.

Yet, incredibly, he has never scored in a World Cup knockout match. All of his six goals have come in the group stage.

He will be 35 by the time Argentina play Saudi Arabia in their opening game on November 22.

"I am going to have to reassess a lot of things after the World Cup, whether it goes well for us or not," Messi admitted in March.

"I hope it goes well, but a lot of things are definitely going to change."

- 'I will decide' -
Ronaldo has also played at four World Cups and last year broke the international scoring record held by Iran's Ali Daei, but he has never managed a goal in the knockout rounds either.

The Portugal captain will be nearly 38 at the end of this year, yet he remains in impressive physical shape and still scored 24 goals for Manchester United this season.

"I will be the one to decide, nobody else," he insisted in March when asked if this could be his last World Cup.

Improved diets and advances in sports science mean more players now are extending their careers at the very top well into their 30s.

Karim Benzema, who turns 35 the day after the final at the Lusail Iconic Stadium, is playing the best football of his career for Real Madrid, the top scorer in La Liga now perhaps the favourite to succeed Messi as Ballon d'Or winner.

So far, Benzema has played at just one World Cup in 2014 before being exiled from the national team for five years over his involvement in a sex-tape blackmail affair.

"There is a World Cup coming and I have to try to do something great. We will see after that if I go down in history," Benzema told L'Equipe recently.

- The new wave -
The best player at the last World Cup, Luka Modric has continued to mesmerise in flashes this season alongside Benzema for Real Madrid.

The 2018 Ballon d'Or winner will, though, be 37 come Qatar.

A repeat of his heroic physical efforts four years ago –- when he dragged Croatia through extra time in three knockout ties en route to the final -- is highly unlikely.

Robert Lewandowski may be the greatest pure goal-getter in European football in recent years, but he has only played three World Cup games for Poland and never scored.

Soon to turn 34, this is again his last chance at the tournament, and the same can be said of Uruguayan veterans Luis Suarez and Edinson Cavani.

Even Neymar, still comparatively young at 30, has said he might not feature at a World Cup again, such is the strain on his body and mind.

"I will play it like it is the last because I don't know if I will still have the mental strength to put up with even more football," the Brazilian said last year.

Yet Qatar may see one of the game's emerging young stars really make their mark, like Mbappe four years ago.

Norway's failure to qualify means no Erling Haaland, but Spain's Pedri, Dusan Vlahovic of Serbia and Vinicius Junior of Brazil are among the new generation of superstars. If only they can outshine the old boys.

Qatar's 'Dr Cool' keeps World Cup stadiums chilly with solar-powered AC

Qatar has become almost a byword for scorching heat, but some fans will still take a sweater to World Cup stadiums because of state-of-the-art air conditioning that its mastermind says will become the norm for mega sports events.

Saud Abdulaziz Abdul Ghani, nicknamed "Dr Cool", worked for 13 years on the solar-powered cooling system that he says will keep the players and turf healthy and even eliminate body odour in a packed stadium.

The mercury can hit 50 degrees Celsius (122 Fahrenheit) during the Gulf state's blistering summers, which is why this year's World Cup was moved to the winter.

But even with maximum temperatures down to around 25 Celsius (77 Fahrenheit) for the tournament in November and December, cool air will still be pumped out onto the players and watching fans.

Stadium cooling is nothing new. The Superdome, home of the New Orleans Saints American football team, has 9,000 tonnes of air conditioning equipment.

"Dr Cool", a professor of engineering at Qatar University who also helped develop cooling for the Ford Mondeo car, has however developed a system that World Cup organisers say is 40 percent more "sustainable" than existing techniques. 

Seven of the eight stadiums are air-conditioned at a World Cup that organisers insist will be carbon-neutral.

At the 40,000-capacity Al Janoub Stadium, which will hold seven games including holders France's first match, Saud said a two-metre-high "completely isolated bubble" of cool air will envelop the pitch and stands.

Inside the bubble, players and fans will be kept at 21 Celsius (70 Fahrenheit) by jets blasting air at the pitchside and under spectators' seats.

Sensors around the stadium keep the temperature constant and even adjust air flows for seats in the shade or sun.

The rising air is sucked back into the stadium cooling system, cleaned by water kept at a brisk 7 degrees Celsius (44 Fahrenheit) and pumped out again by the jets.

"The players will have the best experience of their lives," said Saud, highlighting how the chilled air would prevent injuries and illness suffered in extreme heat.

- Giant solar farm -
The power for the system comes from a giant solar farm in the desert outside the capital Doha, he added. The same technology is being used in greenhouses where Qatar grows increasing amounts of its own food.

"We have the best thermal insulation on our machines, the best sensory systems around the stadium," said Saud.

And the air conditioning will still be needed in December, despite the cooler temperatures.

Each human generates the heat of two laptops and gives off 70 grams (2.5 ounces) of sweat per hour, according to Saud.

He gave the example of the Lusail Stadium where 80,000 people will gather for the World Cup final on December 18.

"They are there for four hours, so that is a lot of water. And I also have the heat of 160,000 laptops in that space. So that heat must be offset irrespective of whether it is winter, summer, autumn or spring."

The use of air conditioning in stadiums remains controversial, however.

Russell Seymour, chief executive of the British Association for Sustainable Sport, said that while the technology and renewable energy in Qatar may work he had concerns about the wider message given by air conditioning an open space.

At a time when people are being urged to save energy "quite often people in an office will open the windows, they want fresh air but they've also got air conditioning on and then things compete, and that's when the issues come".

Saud said he is happy for any expert to inspect the system and check his sustainability claims. The technology has been made free of patent restrictions for anyone to copy.

He is also certain that future World Cups -- particularly in 2026 in the United States, Mexico and Canada -- will follow suit.

"In the future, for the safety of players, air-conditioned stadiums will be more of the norm," he said.

As global temperatures rise due to climate change, "if you want players to complete the game without water breaks, without any interruptions then air conditioning will be a necessity".