Singapore Grand Prix offers drivers their sternest test of the season. Ask any Formula One driver which is the toughest race on the calendar, and without hesitation every one of them will say the Singapore Grand Prix.

The city-state’s floodlit spectacular is the one race on the calendar they spend all of those hours over the winter and through the season training for.

Because, while the race takes place under the floodlights at night, against the glittering backdrop of the Singapore’s iconic skyline, the city-state’s unrelenting tropical heat and humidity can sap the energy and test the limits of even the most hardened of these 20 super-athletes.

“Yeah, when you’re a high-performance athlete… it is, it’s the toughest, sure,” said Red Bull’s Daniel Ricciardo in the build-up to the 2016 edition of the race.

“It’s the one race I feel where you open your visor to get some air and you’re not getting any reward for that.

“It’s just heat...”

Formula One - F1 - Singapore Grand Prix - Marina Bay, Singapore- 17/9/16 Red Bull's Max Verstappen of the Netherlands in action during third practice. REUTERS/Jeremy Lee

Formula One drivers are among the fittest athletes in the world. The physicality of their chosen sport may not be as visible as that of a tennis player, but driving a Grand Prix car to the limit requires exceptional strength, stamina and endurance.

Their speed around corners and short braking distances regularly impose forces equivalent to as many as five times — and occasionally six times — that of gravity on their bodies.

Their heart-rates hover around the 160-180 beats per minute mark for the duration of the race.

To slow their thoroughbred racing machines enough to make the tightest of corners, they have to hit the brake pedal with around 180 kg of force. That’s like lifting 180 kg of weight on leg day in the gym - with just one leg.

And yet when they step out of their cars after most races, they’ve barely broken a sweat.

Not so in Singapore.

Mercedes Formula One driver Lewis Hamilton of Britain smiles as he walks with his assistant in the rain after qualifying for pole position in the Singapore F1 Grand Prix at the Marina Bay street circuit in Singapore September 20, 2014. REUTERS/Edgar Su

As a street circuit, winding its way through the heart of Singapore’s financial district, the track’s layout features a lot of tight slow corners but very few of the fast, high-speed, sweeping corners that place the greatest strain on drivers’ bodies.

But the heat and humidity they face is like nowhere else on the calendar, not even the Middle Eastern desert races of Bahrain and Abu Dhabi.

Ambient temperatures regularly hover around the 30 degree-Celsius mark. Humidity can soar to as high as 80 or even 90 percent.

The average Joe, adding a touch of exaggeration, would describe these conditions as unbearably hot.

But it wouldn’t be an exaggeration to say that for the drivers, clad head-to-toe in helmet and fireproof overalls, it borders on the inhospitable, as ‘ambient’ temperatures in the cockpit can reach a scorching 60 degrees Celsius.

And they have to cope with these conditions for nearly two hours, with the race — easily the longest duration event on the calendar — often running up to that rules-mandated time limit.

“It’s humid, it’s hot and it’s always a long race,” said the Haas F1 team’s French driver Romain Grosjean ahead of last year’s race.

“We usually reach the two-hour limit. It’s very, very demanding. I remember back in 2013, I lost four kilos of water during the race, which is quite a lot.”

If the physical demands weren’t enough to deal with, there is the mental challenge. Driving a street track is like threading the eye of a needle. One slip-up can mean the end of the race for a driver.

Keeping the car out of the barriers under such physical duress, while at the same time adjusting the various settings on the steering wheel, not to mention processing the flood of information they get over the radio, makes the race mentally taxing for the drivers.

Dehydration inevitably sets in and as the body tires, mental fatigue sets in. All this around a venue that demands elevated levels of concentration.

“The last ten laps, twenty laps, you’re really praying for a safety car because it’s just really hard to finish the race, physically,” said Force India driver Sergio Perez in 2016.

“And mentally as well. Mentally it’s a very big challenge because every corner, most of the corners, if you do a mistake you just touch the wall and it’s a puncture or something like that,” added the Mexican.

Formula One - Singapore Grand Prix 2017 - Practice - Singapore - September 15, 2017 Toro Rosso's Daniil Kvyat during practice REUTERS/Edgar Su

To prepare, drivers must drink plenty of fluids in the week leading up to the race. All they can carry in the car is about a litre of water which, far from a cool, refreshing drink, is more like sipping a warm cup of tea in the heat of the cockpit.

During the two-week gap between the Italian and Singapore races, many drivers fly out to Asia early to acclimatise to the heat and humidity. Others will crank up the temperature in the sauna and train hard.

“The night race and hot temperatures really test you to the limit and for me Singapore is physically the hardest race of the season,” said Max Verstappen last year.

“I have been preparing already for a few weeks doing heat training in the sauna and getting ready to sweat.”

Nutrition is equally important, especially as the body’s metabolism changes in the heat.

Physio and nutritionist Josef Leberer, whose trademark blend of breakfast muesli powered Ayrton Senna and Alain Prost to multiple championships with McLaren in the 1980s, knows this.

“You have to plan, you have to think about it and what is possible with food and the right drinks,” said the Austrian, who now works with the Sauber team.

“Light food, vegetables and salads and the right fruit are perfect to get a good balance.

“With some herbs mixed even in water or tea... it’s working quite well.”

Saving energy, avoiding unnecessary activity and of course keeping the body cool and topped up with vital minerals and salts plays a vital role in a driver being able to extract his best on race day.

“You do your training and warm up a little bit, but not overdo it,” said Leberer.

“So we’re saving a lot of energy, this is a key factor.

“And then, of course because of the humidity, we’re losing a lot of liquids, minerals or salts, so to replace this is a very important part.”

Ultimately, the key to success in Singapore lies in the preparation.

“If you come here unprepared, then you can’t luck into a good result,” said Ricciardo, who has finished on the podium in each of the last four years.

“You’ve got to come ready to go.”

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