Hingis is known to be a bit wild so it would not surprised me if she took cocaine. I would not say that she dominated the sport b4 injuries beset her as 1997 was the only year that she was really dominant. Interestingly that was the year where Graf was out for most of the season and the Williamses' and Davenport's game have yet to mature. Once Venus, Serena and Davenport fulfilled their potential from 1998 onwards, Hingis' success began to dwindle.
Hingis relied heavily on her tennis smarts to win a lot of matches. She did not have the size and power to stay at the top. She also seems to lack the motivation to work harder but hey with the way she dominated tennis in 1997, she probably think she does not need to do so.
When Henin first arrived on the tour. people raved about her talent especially her glorious one handed backhand but they questioned if she has the size to win a GS title. Unlike Hingis, Henin put in the work to make herself the 2nd most successful player of the last decade.
Hingis is still the holder of numerous records like the youngest No 1, youngest Australian Open champion. However, even without the injuries, I still do not see her having much success against the powerful hitters who are playing today