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Dunedin's Lars Humer was alongside the 15-year-old as a coach on the New Zealand team at the junior swimming world championships in Budapest.
She had come up with how she planned to approach the 200m freestyle, which she ended up winning in a time of 1min 57.96sec.
It left her just one second off Lauren Boyle's New Zealand record.
It was also enough to win a title at an under-18 level.
Yet it did not come as a complete surprise to Humer.
''I knew Erika was capable of doing it,'' he said.
''She told me before the race started that she believed it would be a 1min 57sec to win.
''I agreed with that, because in training we work on various times and splits and things. We're in tune with that.
''We'd already seen the way the heats unfolded and her words were that she believed she was capable of doing that.''
Humer said it was exciting, not only for Fairweather but for Dunedin and New Zealand swimming.
That she had led the whole way had been the most exciting part and showed she was a deserving champion.
Humer felt she had great potential and being a junior world champion left her in a very good position to push on.
It was a case of continuing to get better, finding a way to improve every day and getting the most out of every training session.
That was something she had done well to date.
''She's very dedicated, very passionate ... she really has a great desire to do well.
''If we're in a training environment and the set doesn't go as well as she would have liked, she gets annoyed at herself - she sets very high standards.
''I think you have to be like that on the world stage.
''And she just really enjoys the sport, loves swimming, loves swimming fast and loves racing.''