Sofia Richie battling sickness on 18th birthday

Justin Bieber's girlfriend Sofia Richie is battling illness as she celebrates her 18th birthday.

The youngest daughter of soul singer Lionel Richie marked the age milestone on Wednesday (24Aug16), but on the eve of her big day, she revealed she wasn't feeling her best.

"Of course I'm sick (the) day before my birthday," she wrote alongside a Snapchat picture taken inside a doctor's office in Los Angeles. "Anything else want to go wrong?"

The birthday girl appeared to be well enough earlier in the day as she enjoyed an outing with Justin, reuniting with the pop superstar after he returned home following weekend (20-21Aug16) performances at Britain's V Festival.

The young couple was spotted hanging out together in North Hollywood after spending the past week apart.

Justin and Sofia went public with their budding romance earlier this month (Aug16), and the old friends promptly took their relationship overseas by jetting to Japan, where the Baby singer was booked for a pair of performances.

However, Bieber's fans haven't been the most receptive to his new girlfriend, and he lashed out at online trolls for attacking Sofia on social media, even threatening to make his Instagram page private if the nasty comments didn't stop.

The backlash even sparked an online dispute between Justin and his ex-girlfriend Selena Gomez, after she criticised the 22-year-old for taking aim at his own followers. The pair dated on and off from 2012 to 2014.

Justin followed through with his Instagram boycott threat just two days later, after the hateful messages continued to roll in.