World number two Williams was outplayed by his Thai opponent who goes through to the last 32 to face Zhou Yuelong. Akani, ranked 65th, has scored some fine results over the past year, notably reaching the last 16 of the 2017 UK Championship, but this is perhaps his best win yet.
Welshman Williams led 3-2 but 23-year-old Akani reeled off the last four frames with top breaks of 108, 101 and 62.
“I was poor all the way through, I don’t know if I can play much worse than that,” said Williams. “Sunny deserved to win. He made some good breaks but he is very slow and methodical.
“Snooker is a tough game and you get what you deserve. I have put nothing into it so I’m getting nothing out. I know I should be practising, but whether I will or not, I don’t know.”
There was no such trouble for Mark Selby as he beat Ken Doherty 6-0 and he will now gain ground on Williams in the race to retain his world number one position. Selby, aiming to win this title for the third year in a row, fired breaks of 100, 55, 124, 117 and 52 as he set up a meeting with Robert Milkins.
Fight-back of the day came from Mark Allen as he recovered deficits of 4-0 and 5-2 to beat Liam Highfield 6-5. In a high-quality match, Allen knocked in breaks of 120, 127, 125 and 85. He now plays Iran’s Hossein Vafaei, who beat Graeme Dott 6-3 with a top run of 136.
Allen, who was runner-up in this event last year, said: “If I had lost 6-1 I would have had no complaints, Liam was awesome at the start. He completely froze me out. At 4-0 I was just trying to be patient because I didn’t even know how I was playing, it wasn’t as if I had missed chances.
“Intervals can change matches and I scored really well to get back into it. I have reached two finals in this event and lost them both so it’s good and bad memories. It would be nice to go one better this week.”
Ding Junhui was pushed hard in a Chinese derby with Zhang Anda but crucially came from 60 points down to win the ninth frame on a respotted black to lead 5-4, then sealed a 6-4 victory in the next. Ding now plays Zhao Xintong, who fired breaks of 104, 137 and 116 to beat James Wattana 6-5.
Neil Robertson was in fine form in a 6-1 thrashing of Matthew Selt, compiling runs of 55, 65, 82, 73, 113 and 55. Marco Fu came from 2-0 and 4-3 down to edge out Andrew Higginson 6-5.
Michael Holt also showed his break-building skills as he made 107, 140 and 106 in a 6-4 win over Luo Honghao. Judd Trump top scored with 123 in a 6-0 whitewash over Alan McManus.