With the temperature topping 33C and the humidity approaching 50% - and that combination making it feel more like 40 C - nearly everything became a struggle for every player across the grounds.
Sharapova let a big second-set lead slip away, then needed four match points to close out Schnyder 6-2, 7-6 (6).
For the first time since Wimbledon in June 2017, a tournament will have the entire Big Four in the field: Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray.
Rafael Nadal and Serena Williams, the two superstars, who own 40 Grand Slam titles between them, headline an opening day that sees eight former US Open champions in action.
The 21-year-old German, ranked fourth in the world, is part of the highly-touted "NextGen" crop of 20-somethings who are making an impact on the ATP Tour.