Kartal watched GB’s triumph against France in last year’s qualifiers from afar, having spent three months out with a health issue.
However, she has risen up the rankings in the past 12 months, clinching her first WTA title on the way.
A good mover on the clay surface, she played confidently in a first set that saw seven breaks of serve in 10 games, adjusting well to Niemeier’s pace.
Kartal served for the opener at 5-3 but was broken, before two double faults from Niemeier handed the Briton the set.
Germany’s Niemeier, heavily beaten against the Netherlands and struggling for form, covered her face with her towel at the end of the set as she became visibly upset.
Kartal reeled off five games in a row to lead by a set and a break, redirecting the German’s pace around the court, before serving out victory.
“I’m super proud of that debut match,” said Kartal.
“It is obviously nerve-wracking when you’re the first match on, but I’m proud of my performance.”