He walked in the park, met friends, got fit and thought about his future. When he returned to the concert hall in July last year, he says he felt like a newborn baby. It was to a much-reduced schedule.
He is now doing 70 recitals a year instead of 130, with three days in between each concert instead of one. "This will give me time to heal, mentally and physically."
Familiar though it is, the Lang Lang CV still astonishes. Inspired by watching Tom and Jerry's cartoon escapades in The Cat Concerto, featuring Liszt's Hungarian Rhapsody No 2, he started playing the piano at three.
At five he gave his first public recital in his hometown of Shenyang, in industrial northern China.
"The stage felt like a sweet home to me," he wrote in his 2008 autobiography Journey of a Thousand Miles.
"Right at that moment I decided to be a concert pianist."
His father Lang Guoren - whose own musical ambitions had been thwarted by Mao's Cultural Revolution (which outlawed the playing of Western classical music) - sacrificed everything to make this happen.
He gave up his job as a policeman and moved with his son, when he was nine, to Beijing, so that Lang Lang might have the chance of studying at the Central Music Conservatory.
The pressure was immense. Father and son lived hand-to-mouth in Beijing while Lang Lang's mother worked in Shenyang as a telephone operator to pay for his music lessons.
At 10, the boy entered the Conservatory. By 13, he had won first prize at the Tchaikovsky International Young Musicians' Competition.
At the Curtis Institute of Music in Philadelphia he studied under Gary Graffman, the country's leading music professor, and became an overnight star at 17 when he performed with the Chicago Symphony Orchestra as a last-minute replacement for an indisposed soloist.
Lang Lang has used his subsequent fame and fortune - he is said to be worth £22 million - to bring music into the lives of children through the Lang Lang International Music Foundation, through two piano academies in China, masterclasses and a series of books that teach and popularise the piano.
The so-called Lang Lang Effect is said to have produced 60 million aspiring young pianists in China alone.
"Piano is becoming more global these days," he says. "The passion is incredible."
His own teaching methods are more motivational than academic. "I just start playing," he says.
"I don't talk much because the students already get enough talk. They might want me to play a Lady Gaga song or some Mozart. Or we just compose together."
At the end of the session, there are high-fives all round, a game of ping pong, maybe some sport.
"We wanted to find a better way for them to overcome the things I struggled with when I was a kid. I believe people will learn music in a slightly more joyful way. I like to find ways to spark their imagination and let them discover their own fantasy. Some teachers are very narrow-minded and just say, 'Do this, do that.' You cannot teach kids in that way...
No one is a machine. Especially when you are a kid, you are so sensitive."
His latest album, Piano Book, is a touching collection of miniature pieces that made him want to become a musician in the first place, including folk tunes from different cultures.
His approach is so far from his own experience as a child that you wonder if it's partly a reaction to his father's harsh regime."I wouldn't say that. For sure, my father was pushing quite hard but that wasn't everything. I also had other encouragement. My father is not always like a dictator.
Sometimes he's quite nice."Pushing "quite hard" is a breathtaking understatement about a man who used to make his son practise scales and exercises for hours on end and tell him over and over again, "You must be number one, you must be number one."
On a day that is now a notorious part of the Lang Lang legend, he once ordered his son to commit suicide. His piano tutor had vindictively announced just before Lang Lang was due to take the entrance exam to get into the Conservatory that she would no longer train him, he was a lost cause, and Lang Guoren had taken the news badly.
When Lang Lang came home late from school, his father flew into a rage. "You've missed nearly two hours of practising and you can never get them back. It's too late! Everything is ruined! Dying is the only way out."
Lang Sr then told his son to take an overdose and, when Lang Lang fought back, ordered him to jump off the 11th floor balcony.
The boy brought Lang Sr to his senses by hammering his fists against the wall until they bled, shouting, "I hate my hands!"
Does the pianist feel any lingering resentment towards his father?
"That was a long time ago. I was nine. Anyway, it wasn't completely his fault because I had this professor who liked to torture us and I think [my father] just went nuts."
Lang Lang understands now the mixture of love, hardheadedness and ambition that drove his father.
"Unfortunately, I had to work harder than others. I am not from a rich family. I had no connections. Without those experiences, I may not even have crossed the road from Shenyang to Beijing. To get to where I am today was not easy."
Lang Lang is courteous and composed, even a bit stiff at first, despite the casual Louis Vuitton jacket and black T-shirt. His trademark off-centre quiff looks as if it has been sculpted by a high wind. I find his fingers mesmerising - long, pliable and blunt-ended with small but prominent knuckles. When he plays a virtuoso piano piece they blur to the point of invisibility.
His hands are so precious as to be almost uninsurable. "I don't play very dangerous sports. I put gloves on, keep my hands warm... try not to fight people."
Some critics feel his extravagant stage style can get in the way of nuance and subtlety, but he argues that his extrovert way of playing is just his way of expressing himself - and has been since he was a boy.
"I am quite a physical player, and I feel pretty good about it. I am not a stone, I am a musician."
Adverse criticism stings, but not for long. This is a man who can fill concert halls all over the world and is reportedly paid up to £170,000 for private recitals.
He has homes in New York, Paris and Beijing, gives his name to high-end merchandise (currently Hublot watches) and was in Time magazine's annual 100 most influential people in the world.He has a huge young fan base - 239,000 followers on Instagram and 14 million on Weibo, the Chinese Twitter.
His room in London's five-star Landmark Hotel, where we meet, is equipped with a Steinway so he can practise whenever he likes. Lang Lang's mother Zhou Xiulan is with him on this trip. After years of separation from her son, it is her turn to see the world while his father stays in China, looking after his schools.So he can afford to be dismissive, but isn't.
Critics, he believes, are a necessary reminder, especially to young musicians, not to get carried away.
"When you are flying high and you think you're invincible, it's great that someone says your interpretation is pretty bad. You have to pay attention: Don't be so proud of yourself - you're just another young man, watch your step. That's good. It gives my music focus, makes me more solid."
Though firmly in the classical tradition, Lang Lang sometimes collaborates with musicians from other genres - the jazz legend Herbie Hancock, Pharrell Williams, Metallica, Julio Iglesias.
He says they keep his mind open and enrich him on a personal as well as a musical level. He bridges East and West with an eclectic repertoire that encompasses film soundtracks, new works, Chinese music, as well as the great Western classical canon. Though he has no desire to become a crossover artist, he doesn't look down on others who are.
"I don't believe classical music should be played for what you call an elite. The 'elite' also like to listen to rap."
Known details of Lang Lang's personal life could so far be written on a postage stamp. Shuttling between planes, concert halls and hotels leaves little space for deep friendships to take root.
But being suddenly exiled from the concert hall by illness has given him time for reflection about marriage and children and he is in a steady relationship with a fellow musician, Gina Redlinger.
According to reports last year, the two are engaged. The Lang Lang autobiography could soon be due a radical update.
