H2H is the tiebreaker for 2 people tied in number of wins. If 3 people are tied then number of sets/games won comes into picture. Thiem has beaten Tsits and rafa bothSomeone correct me if I'm wrong, but he hasn't secured the top spot, only a place in the semis.
If he loses in straights today and Nadal wins in straights, wouldn't Rafa be #1? They would both be 2-1 win/loss but Thiem would be 4-3 in sets and Nadal would be 4-2.