Japan's Kento Momota was once badminton's undisputed king but his career has gone into free fall ever since a car crash more than two years ago which left him badly hurt.

There are now severe doubts that the 27-year-old former two-times world champion -- who was embroiled in a gambling scandal earlier in his career -- will ever reach the top again.

Momota's world was turned upside down in January 2020 when the vehicle taking him to Kuala Lumpur airport crashed hours after he won the Malaysia Masters, killing the driver and leaving the player with a fractured eye socket.

He was out of action for a year and is now a shadow of his former self -- the left-handed ace flopped at the Tokyo Olympics and has lost in the first round of four of the five singles tournaments he has entered this year.

"His defence is still a bit dubious, his net game isn't as strong as it used to be and then of course his self-confidence must have taken a big, big hit," former Danish national team coach Steen Schleicher Pedersen told AFP.

"Maybe even his life values, after being in a crash like that where the driver died. Maybe he doesn't have that hunger any more, maybe he doesn't care, maybe it's changed him," added Pedersen, now a badminton commentator.

Pedersen said it looked like Momota's vision was affecting his play last year, especially his defence.

Momota won a record-breaking 11 titles in 2019, losing just six of the 73 matches he played and looking every inch a legend in the making.

Then came the crash which left him with double vision and required surgery on a bone near his eye.

Home hope Momota flamed out in the first round of last year's pandemic-delayed Tokyo Olympics, but he looked back to something like his best when he won the Indonesia Masters in November without dropping a game.

A back problem then forced him out of the following month's World Tour Finals and world championships and he has struggled to generate any momentum since the new year.

Japan's Kento Momota has lost in the first round of four of the five singles tournaments he has entered this year
Japan's Kento Momota has lost in the first round of four of the five singles tournaments he has entered this year AFP / Ted ALJIBE

The enigmatic Momota has publicly said little about his fitness and Pedersen says the lack of information means those on the outside must rely on "guesswork".

"Has he been able to practise fully this year? If he's been limited in his practice, that could be a part of the explanation," he said.

"If he's practised 100 percent from January 1 to now and he's been hitting the gym and doing all the badminton practice that he needs and this is the outcome, then there's no more, most likely."

Momota is still ranked number two in the world but the points system has been skewed by the coronavirus pandemic and he is expected to plummet once a truer picture emerges later this year.

He did not sound confident when he spoke to Japanese reporters in April, saying he just wanted "to do all I can at this moment".

"It makes me sad when I see comments like 'he's not playing well' or 'the Momota of old doesn't exist anymore', but I'm trying not to pay too much attention to that," he said.

Momota suffered another first-round defeat on Wednesday, this time to Danish world number 13 Rasmus Gemke at the Indonesia Open.

He looked well off the pace again and said afterwards that he "didn't have the stamina", blaming himself for making "so many mistakes".

Momota has bounced back before and will need to do so again if he is to be a force at the 2024 Paris Olympics.

Momota was banned from Japan's 2016 Rio Olympics team for gambling at an illegal casino.

The ban forced him to play in smaller tournaments around the world as he tried to climb back up the rankings, and sent him on a winning run that took him all the way to the top.

Pedersen says playing regularly "gives you confidence, it gives you touch, it gives you accuracy", and he believes Momota "definitely needs to start winning".

"He needs to do good practice and then he needs to start winning matches -- a lot of matches," he said.