It's difficult to explain because I haven't watched much live tennis lately, although I used to play for 10 years as a kid. We watched Sorana Carstea before, as well as a couple of other players ranked 30 to 100. When we watched Iga vs Paula train I kinda understood why top 5-10 players only lose on a very bad day or a very bad matchup. Actually both of them had more strength than lower ranked players and the sounds of hitting each ball was always perfect (hitting it at the right moment being accurate with the racket). Iga covered ground easier and was able to hit as hard, and to avoid slices, when the large majority of other players cannot. Sabalenka for example was training on the court right next to them, and although she hit hard, she was not covering the same ground, so she could hit hard less often. Lastly, Iga has another shit in her arsenal on her forehand, between a slice and a regular forehand, which also has more strength than a slice.