It usually cools down fairly substantially in the evenings/nights, the temperature differences between night and day being more extreme than in more temperate climates. Added to that the cooling effect of the almost daily afternoon downpours, and evenings can even get chilly...

But not sure how that effects tires on the runway, which I imagine would get hot regardless of the ambiant temperature.

FYI, KLM does fly 777s daily AMS-JNB without a problem (I assume) and they are packed pretty full (no first class, business class has less seats than on BA if I remember correctly).