Nikon recently stated that high end DSLR sports camera are still superior but this might not be true anymore:
The new Sony A9 5.0 firmware has been tested by Tony Northrup. He now states the A9 AF tracking is better on the Sony A9 than on the Nikon D5. Mirrorless finally did beat DSLR in their last bastion.
And here two more videos:
Sony a9 video tracking fix
Sony a9 Firmware 5.0 Menu walkthrough