The 18-year-old was involved in the accident in the early hours of Thursday morning, where firefighters tried to rescue Emilio near the Villas do Mar estate.
According to Portuguese TV channel TVI24, the tragedy happened in Troia, a peninsular off the coast of Setubal just south of Lisbon, close to the house of Ballack, who retired as a professional footballer in 2012.
The quad-bike was designed to be used on public roads.
Emilio is one of three children of Germany legend Ballack, who married long-term girlfriend Simone Lambe in 2008 before the couple divorced four years later.
He was born in 2002, a year after their eldest son, Louis, and was three years older than Ballack’s youngest, Jordi.
On July 25, Emilio’s older brother Louis took to Instagram to post about how ‘quads were flipped, ferries were missed, taste buds where explored and injuries were common’ along with pictures of a day him enjoying down downtime in Troia.
Emilio recently posted pictures of his time in Portugal at the weekend, showing pictures of his time at the beach with his family.
The teenager had been spending time at Sublime Beach Club in Alentejo a few days ago – a trendy hangout area near the scene of Thursday morning’s crash.
Details of his accident have not yet been disclosed.
Ballack is widely regarded as one of the best German players of all time and is a Premier League and FA Cup winner with Chelsea.
Chelsea tweeted on Thursday afternoon: ‘Everybody associated with Chelsea Football Club is shocked and saddened to learn of the death of Emilio Ballack at the terribly young age of 18.
‘All our thoughts are with his father Michael and his family at this sad time.’
Ballack joined the Blues in 2006 after making his name at Bayer Leverkusen and Bayern Munich before spending four years at Stamford Bridge.
Emilio Ballack (left), son of former Chelsea footballer Michael Ballack (right), has died after a quad-bike accident
Emilio, 18, pictured in his last Instagram post at the family’s holiday home just six days before Thursday’s accident in Portugal
On July 25, Emilio’s older brother Louis took to Instagram to post about how ‘quads were flipped’ during downtime with friends in Troia, Portugal
The 18-year-old was crushed by a quad-bike near the family’s holiday home in Portugal, according to reports
Ballack’s sons Louis (left), Emilio (second right) and Jordi (right) pose for a picture with their mother Simone (centre left)
According to Portuguese media, the tragedy happened in Troia, a peninsular off the coast of Setubal just south of Lisbon
Ballack made 98 appearances for Germany, scoring 42 goals in the process.
After joining Chelsea for £18million in 2006, Ballack went on to make 167 appearances for the club, scoring 25 goals and registering 24 assists. It was the longest he had spent at a club.
During his time in Germany, Ballack won the Bundesliga three times, along with the DFB-Pokal all in the same year.
During an interview with the Independent in 2009, Ballack revealed how he saw similarities with his own schooling in what was then East Germany.
He said: ‘You had to stand up in school and say some things and stand still, and wear some uniform.
‘Now I come to England and again my kids are in uniform, in a different way but they have to wear uniform. In West Germany you go to school everybody wears their own stuff.’
Ballack recently had a tumour removed from near his spinal cord after suffering a health scare during the lockdown period.
The former midfielder had a growth discovered on his body during an MRI examination by doctors, according to Bild.
But the early detection proved fortunate for the 44-year-old, and the tumour was removed.
Laboratory examinations then cleared Ballack of any fears that the growth was cancerous, with results showing it was benign.
He recovered in Portugal five weeks on from the operation and would return to work as a pundit.
More to follow
Emilio (left), who has an older and younger brother, pictured playing a round of golf with his dad Michael back in 2015
Chelsea icon and Emilio enjoy a day out in New York City in 2019, where they spent some downtime in Times Square (right)
Ballack, pictured with sons Louis, Emilio and Jordi, at the World XI testimonial game at the Red Bull Arena in Leipzig in 2013
Ballack waves to the crowd at his testimonial as he walks around the pitch with Jordi, Emilio (second left) and Louis in 2013
Ballack spent four years at Chelsea from 2006 to 2010 and lifted one Premier League title during his time at Stamford Bridge