Or even the Power Of One using Bruce Lee as a prime example.
I have different view from this.
Let me evaluate why.
While it is best to master one skill at a time, it is not best to stick to that one skill if you are already a master.
Some people tend to get emotional as in being proud and even complacent in what they have achieved -
That they refused to stay open and learn new skills that can help them to be better.
Truth being said, as years come, changes are inevitable.
Unless what you do has a high perceived value to the society like chefs, doctors, drivers e-commerce, you have to be open to learning and mastering new skills as technological evolution comes about.
Jobs like cleaning, corporate, insurance, properties and even retail will be soon be a thing of the past.
Here is my formula.
It is called Pomodo - a Japanese technique I learn from established manga and story writers.
It means mastering one skill at a time and once you are really good in it, go and master another skill.
Though being a master of one is good, it is better to be a master of 1, 2 and even 3 should what you know does not work in case.
Germany and Italian national teams are the prime examples given their performances of defending and counterattacking as a team in recent Euro 2016 matches.