BERLIN, July 18 (Reuters) - Zalando, Europe's biggest pure online fashion retailer, saw sales growth slow in the second quarter, while profitability improved as it launched a membership scheme seen as helping it fight a rising threat from Amazon.

Zalando reported preliminary second-quarter sales rose 19-21 percent to 1.091-1.109 billion euros, while adjusted earnings before interest and taxation (EBIT) was 80-86 million, compared with Thomson Reuters SmartEstimates of 1.1 billion and 85 million, respectively.

That marks a slowdown from the 23 percent rise of the previous quarter, but co-chief executive Rubin Ritter said the company continued to deliver on its medium-term forecast for annual sales growth of 20 to 25 percent.

The Berlin-based company that delivers 1,500 brands in 15 countries in Europe is set to report full results on Aug. 10. (Reporting by Emma Thomasson; Editing by Tom Sims)