Similar to Tiger '12 achieving the #1 rank will take quite a few wins to overtake the #1 spot. You will more than likely obtain this by the end of your 2nd season if you play and win every tournament available. Turning down the AI scoring difficulty when setting up the season settings will make winning very easy to achieve. You then simply need to keep winning until your rank surpasses 50, 10 and eventually the #1 spot. When you are in positing to take the #1 spot you will play against Tiger for a round(s) of the next tournament. You will of course need to win to overtake the position. It has been reported by many and happened to me as well that this achievement will not pop until playing and winning the following tournament after beating Tiger. The good news is unlike Tiger '12, you do not have to maintain the #1 after unlocking the achievement.