Running to the maximum two hours duration and in 29C heat with high humidity, the street-circuit race is renowned as being hard on the drivers.
Singapore Grand Prix driver ratings
Magnussen looked visibly exhausted following the 61 laps in which he secured only the team's second points-scoring finish of the year by claiming 10th place from 15th on the grid.
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"Physically it was very tough, I didn't have any water in the car, so it was pretty tough," Magnussen told Autosport.
"I don't know what the problem was. I'm not sure if there was any water in or if the bottle failed, but the team definitely owes me a drink!
"In the end I just had to block it from my mind and get on with it."
Magnussen was one of only a handful of drivers not to use the soft tyre during the race, which he feels was crucial to him finishing in the top 10.
Daniil Kvyat, who finished ninth, was another of the group of five while the Toro Rosso driver and Sergio Perez in eighth were less than 10 seconds ahead of Magnussen at the end of the race.
"I had a great start, I really nailed that one, making up a few positions from the [startline] crash," the Dane added.
"From there I was able to take care of the tyres pretty well. The key to the race for me was the strategy.
"The team noticed we were good on the tyres, and decided to go for a double super-soft stint at the end, which was a tough call to make.
"The balance of the car was good so we went for a two-stop strategy using ultra-soft and super-soft, which should be really impossible, but they did it and it paid off in the end.
"With a bit more straightline speed, we could have maybe done the Toro Rosso and the Force India in the end.
"But still, I'm really happy with this. The team did a really good job over the weekend and everyone in this team deserves it."
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