Five-time champion Ronnie O'Sullivan marked his return to major tournament action with a battling 6-5 victory over Mark Williams in the first round of the Masters at Alexandra Palace in London.

The 40-year-old had taken some time away from snooker following the World Championship in April, where he lost in the quarter-finals to Stuart Bingham who went on to win the tournament at the Crucible.

O'Sullivan had opted not to defend either his Champion of Champions or UK Championship titles as he battled debilitating insomnia as well as issues with his back.

The Essex player started confidently to move 2-0 ahead, but then Williams responded as the Welshman opened up a 4-2 lead.

Five-time world champion O’Sullivan – who did play in the qualifying for the German Masters during December, but failed to make the tournament stage – then rattled off two century breaks as he claimed the next three frames to edge ahead 5-4.

Williams, however, was let back in to win the 10th frame and force a decider.

The twice Crucible champion, though, failed to pot a plant when looking to build a break at 28-0 and O’Sullivan closed out victory with a run of 62.

“Unless you are playing really well, it is difficult to win tournaments,” O’Sullivan said afterwards. “I should have lost that match today really, but I got lucky. Mark will feel disappointed about losing it.

“I didn’t really feel nervous. When I was 4-2 down, I had nothing to lose and I loosened up a little.”

O'Sullivan, who will face either Mark Selby or Ricky Walden in the last eight, added: "I missed a couple of tournaments due to problems with my back. Unless I play well and start flowing a bit, there is no joy in it for me.

“I am miles away from my best, but I am pleased to be in and I hope to gain some momentum.”