"I think I got what everybody else is having here in Madrid has, unfortunately," Gauff said, referencing the sickness that's forced nearly a half-dozen players to withdraw from the event.

The ATP's No. 1 starts his quest to become the first man to win five straight Masters 1000s since the format was introduced in 1990.

Two dominant No. 1s descend on the Caja Magica, but how will the field fare? Six things to watch.