If two players are tied, then: Head-to-head round-robin result.
If three players are tied, then the following tiebreakers are used, in order, until all three players are no longer tied OR until only two players are tied, at which point the two-player tie is broken by the head-to-head round robin result:
Highest % of sets won. Highest % of games won. Highest ranking at the start of the tournament.