Boulter struggled on serve in the opening two sets, winning just 60% points behind her first serve, hitting four double faults, and facing seven break points, but grew into the match, finding success when stepping inside the baseline to put pressure on the 23-year-old Croat.
Broken in her opening service game, Boulter was gifted a route back into the first set at 4-2 when Ruzic double-faulted on break point. However, Ruzic immediately hit back, winning the next four games.
Boulter wrested back control in an increasingly scrappy second set, saving two break points in the third game to avoid going a double-break down before coming up with a crucial hold in the seventh after the pair had exchanged three successive breaks.
That hold proved the second of eight straight games for Boulter, who raced into a 4-0 lead in the decider. Although she failed to serve out the match at 5-2, she broke her opponent to love to wrap up the win.