Why are Australian Open fans shouting "Siuuu!" at tennis players? Cristiano Ronaldo's celebration explained

Tennis players continue to think they are being booed by the Australian Open crowd. Turns out they are chanting soccer star Cristiano Ronaldo's "Siuuu!" celebration.

source https://www.sportingnews.com/us/soccer/news/australian-open-fans-siuuu-tennis-cristiano-ronaldo-celebration/1pdcr99h6o7xy1ptatbzpna3gl