BRUSSELS, July 12 (Reuters) - Alphabet (GOOGL.O) unit Google will seek to overturn a record 4.34-billion-euro ($5.15 billion) EU antitrust fine at a five-day hearing in September at Europe"s second-highest court, people familiar with the matter said. The European Commission in its 2018 decision said Google had used its popular Android mobile operating system to thwart rivals, an anti-competitive practice dating from 2011. Android, used by device makers for free, is found on about 80% of the world"s smartphones. The case is the most important of the EU"s three cases against Google because of Android"s market power. Google has racked up more than 8 billion euros in EU antitrust fines in the last decade. The hearing will kick off on Sept. 27, the people said. Google did not immediately respond to a request for comment. The Luxembourg-based General Court said it could not confirm the hearing as the dates have not been made public for now. The five-day event is longer than average but not unusual. Google is backed by lobbying groups Application Developers Alliance (ADA) and the Computer & Communications Industry Association (CCIA), Android device makers Gigaset Communications GmbH and HMD Global Oy, which is the exclusive licensee of the Nokia brand for phones, and Norwegian tech company Opera Software. The Commission has the support of the European Consumer Organisation (BEUC), German publishing groups VDZ and BDZV, Czech search engine Seznam, lobbying group FairSearch and French search engine Qwant. The case is T-604/18 Google vs European Commission. ($1 = 0.8422 euros)