He's playing Chance Kornuth, a match notable because Chance will get a million bucks for winning the match, but only give up $250k for losing (plus whatever is won/lost in the match itself).

Phil was consistently ahead for awhile, but Chance has popped him twice in a row, and now Chance is in the black.

https://twitter.com/RunItOncePoker/status/1318845915705245696