Formula on!
The on-off pair spent two days holed up at a five-star hotel in Monaco before emerging arm in arm, looking loved-up.
A source said: "They looked so happy, it was sweet."
Lewis, 27, and Nicole, 33, split in October after three years, saying their work schedules kept them apart.
They were seen a month later hugging awkwardly in Los Angeles but two weeks ago were spotted sharing an intimate meal.
On Thursday, they took a private jet to the French Riviera after Nicole finished a gig in Dublin.
Lewis had gushed on Twitter from the stage-side: "She killed it tonight!" On Saturday, Lewis took Nicole sightseeing on the French Riviera on a rented motorbike. She jetted back to perform last night at London's Hammersmith Apollo.
The source said: "They are taking things slowly and trying to avoid the same mistakes.
"They're making sure they get every moment they can together."