Australia's Neil Robertson became the fifth former champion to crash out in the opening round of the Betfred World Championship as he lost to Robert Milkins in Sheffield.

Robertson threatened to stage a comeback when he won the first two frames of the second session at the Crucible Theatre to reduce his deficit to 6-5.

But the off-form left-hander scored just 36 points in the next four frames as Milkins made breaks of 69, 55 and 54 to secure a 10-5 win and book his place in the last 16.

Milkins, who also beat Robertson in the first round in 2013, told the BBC: "This is the biggest out of the two.

"I can't remember much about five years ago. I don't think I played brilliant and I think Neil was a bit drained at that time. This time I put in a good performance, especially in the first session.

"There was times out there today in the second session I didn't know what I was doing and I was lucky that I just stepped up the pace a bit. I felt I was playing a bit slow.

"Neil won the first two (frames) and was unlucky in the next when he had a massive kick and it sort of changed after that."

Robertson joins former champions Mark Selby, Stuart Bingham, Graeme Dott and Shaun Murphy in making an early exit, a fate avoided earlier in the day by two-time winner Mark Williams, who produced a total clearance of 140 on his way to a 10-5 win over Jimmy Robertson.

The 43-year-old Welshman held a 7-2 lead after the first session, but saw Robertson win a scrappy opening frame and then steal the next with a 55 clearance after Williams had been first in with a break of 54.

However, Robertson then paid the price for missing a tough opening red along the cushion in frame 12 and Williams took full advantage with a total clearance, before moving within a frame of victory with a break of 100 in the next.

Robertson kept his hopes alive by winning the 14th frame but Williams was not to be denied and secured the victory to set up a last-16 clash with Milkins.

Four-time champion John Higgins holds a 6-3 lead over Thepchaiya Un-Nooh ahead of their final session on Wednesday evening.

Higgins took advantage of a nervous start from Un-Nooh to open up a 4-1 lead, before the Thai left-hander found some form to reduce the gap to a single frame.

However, Higgins potted 13 reds and 13 blacks in the eighth frame before running out of position and missing a tough red in his bid for a maximum, while a break of 59 helped the Scot claim the final frame of the session.

Judd Trump also enjoys a 6-3 lead over Chris Wakelin in a match which concludes on Thursday, but it was Wakelin who produced the highlight of the contest with a break of 141, the highest of the championship so far.

That followed a break of 95 in the previous frame as Wakelin battled back from 3-0 down, but Trump won three of the last four frames of the session with contributions of 64, 72 and 51.

The match between Anthony McGill and Ryan Day also continues on Thursday. Day holds a 6-3 lead.