What is surprising is that no one seems to be taking control and dominating the tour. At this stage, you would've expected either Zverev or Tsitsipas or Thiem to show signs that they are the ones everyone should be chasing.
So far, they have disappointed. Zverev is disappointing. He was such a promising player having won WTF against Novak. Thiem had a great run at Indian Wells but he bombs out first round in Miami. Same for tsitsipipas.
They show great promise, make great runs but they don't manage to keep it up like Fed or Nadal or Djokovic.
The depth of field is obvious there, so many good players in WTA at the moment, but a few players are standing out - Kvitova has already made 3 finals I think and obviously Andreescu is really causing a lot of (justifiable) fuss with a mixture of guile, variety and power.
The stranglehold is still holding for the Slams: 87% since WB '04 (39 of 45).
Regarding Masters, the Big 4 had spoilt us with their inhuman consistency.
From 2005 to FO 2019, they'd won 68% of all Masters (36 of 53), probably higher if you include WTF and back gardens ATP500s (Barcelona, Halle, Beijing). Not normal.
Now we're simply seeing more human players, with peaks and troughs.
Isn't going back to the 90s what a good chunk of the posters wanted anyway?