Complex sentence: The group phase of the Champions League is contested as a double round-robin, as are most basketball leagues outside the United States, including the regular-season and Top 16 phases the Euroleague.
Split into simpler sentences: The group phase of the Champions League is also contested as a double round-robin. Most basketball leagues outside the United States are also organized in this manner, as are the regular-season and Top 16 phases of Europe's continental club competition in that sport, the Euroleague.