I walked by the stadium at about 3 PM yesterday on my way to Audi Field for the Defenders home opener. The ballpark looked to be about 30-40% full (announced was 24,765).
The Defenders announced crowd was about 15k.
I know it was a full sports day with the Caps also playing in the evening, and the Defenders game probably hurt Nats attendance a bit, but if the Nats aren't drawing on a Sunday afternoon against Philly, they are in trouble. Seems chickens are coming home to roost with the poor on field product. The game experience at Audi Field absolutely blew away the experience at Nats Park in terms of crowd engagement and excitement (maybe Nats park needs a beer snake and lemons?)
BTW, the Navy Yard area was pretty crowded as we walked from the Navy Yard metro. Most of the restaurants were full with a lot of people cheering for the women's basketball final.