The Formula 1 star and the 'X Factor' judge - who have been dating since 2007, but briefly split in 2010 - marked the occasion by taking a romantic helicopter ride over the Italian island of Capri, where they spent the weekend holidaying with their bulldog, Roscoe.
Lewis posted a picture of their special day on his Twitter account with the caption: ''Me & my lady celebrated our 5 years Anniversary in Capri, a time I will never forget. I love you baby (sic).''
The lovebirds' glamorous trip comes a week after the 34-year-old singer attended the Monaco Grand Prix to show support for her beau, who finished in third place.
The British-born racer was slammed by five-time Grand Prix winner John Watson and told to take his racing career more seriously after Lewis asked if he could take his pet pooch on the formula one paddock.
John said: ''There is no place for a dog in the F1 paddock. For one thing, it is not fair on the animal. It might be in luxurious surroundings, but an engine firing up is like a firecracker or bomb going off for a dog. It's not right.
''Lewis has to decide if he wants to be an F1 driver or a hip-hop star. At the end of the day, you have got to respect the job that you are doing.''
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