The Jumbo-Visma rider won by 49 seconds on GC from his closest rival, Portuguese Rui Costa (UAE Emirates).
European time trial champion and new hour record holder Victor Campanearts came 13 seconds back on Roglic's time in the final stage.
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In a hat-trick of wins, Roglic was also the tour's sprint champion, as he prepares in fine form for the 2019 Giro d'Italia.
Team INEOS' Filippo Ganna came in third for the stage, while Geraint Thomas took the last podium place in GC. Thomas squeezed onto the podium just a second ahead of Felix Grosschartner (Bora-Hansgrohe) after trailing him by two in the tiam trial. David Gaudu (Groupama-FDJ) took second.
Highlights: Roglic seals Romandie GC win with time trial tiumph on Stage 5
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