His achievement is even more astonishing with his victory in Wales coming in only his seventh ranking tournament appearance of the season.
I’ve never been driven by number one, because if I was I’d enter more tournaments. To get to number one playing half the tournaments as everyone else is probably one of the best achievements in all sport.
He also draws level with Stephen Hendry’s record of 36 ranking tournaments on the sport’s all-time list.
O’Sullivan was forced to battle hard against Robertson, who produced a much grittier performance after losing 10-4 to the Essex player in the Players Championship final in Preston’s Guild Hall a fortnight ago.
Robertson had levelled the final at 10-10, having trailed 6-3, but the pressure merely refocused O’Sullivan who claimed the penultimate frame by pouncing on Robertson missing a black off the spot with a an immaculate closing clearance of 89.
O'Sullivan also rolled in a memorable contribution of 129 to lead 9-8 after the third session resumed with Robertson restoring parity at 8-8 from trailing 5-3 overnight.
It will boost O'Sullivan's hopes of winning a sixth world title at the Crucible, and he is certainly optimistic his form gives him every chance of succeeding in Sheffield.
"Congratulations to Neil, him and Judd (Trump) have been the two best this season along with myself. If one of us doesn't win the World Championship I'll be surprised," said O'Sullivan.
This guy (Robertson) has a Rolls Royce cue action.
O'Sullivan lost to Mark Davis in the English Open semi-finals, and Judd Trump in the final of the Northern Ireland Open in his first two ranking tournaments of the season.
But he won the UK Championship for a seventh time before claiming the Players Championship and Tour Championship to usurp Selby at the top. He won the Shanghai Masters, the Champion of Champions and lost to Trump in the Masters final in the three key invitational events of the calendar.
O'Sullivan has won 38 of the 43 matches he has played this season in an astonishing show of consistency that suggests he is improving at a time when players traditionally appear to be on the wane.
Neil Robertson fought back from 6-3 behind to level up the Tour Championship final at 8-8 with Ronnie O'Sullivan to set up a grandstand finale in Llandudno.
With the final session at 7pm on Sunday evening at Venue Cymru, the first man to 13 frames claims the title and a £150,000 winner's cheque.
Victory for five-times world champion O'Sullivan would see him draw level with Stephen Hendry's record of 36 ranking titles on the all-time list and return to world number one after a nine-year absence.
Resuming 5-3 clear from Saturday night, O'Sullivan started the second session with a timely ton to extend his lead to three frames at 6-3.
Ronnie O'Sullivan of England reacts during the final match against Neil Robertson of Australia on day five of 2019 Coral Tour Championship at Venue Cymru on March 23, 2019 in Llandudno.
But he could not move further ahead against an opponent he defeated 10-4 to win the Players Championship in Preston a fortnight ago.
Robertson scrapped his way to the tenth frame to close to 6-4 behind before a brilliant run of 106, his first century of the final, saw the Melbourne player move to within one frame of the world number two.
A 55 helped O'Sullivan move 7-5 ahead at the mid-session with the next frames shared as an 89 from O'Sullivan allowed him breathing space at 8-6 clear.
Despite his advantage, there was a sense that Robertson's matchplay skills were having an effect on O'Sullivan, who wasted several chances among the balls as Robertson drew level at 8-8 by winning the final two frames of the session with trademark grit.
A sign of O'Sullivan's growing discomfort was evident when he conceded the final frame of the afternoon despite needing only one snooker.
Ronnie O'Sullivan opened up a 5-3 lead over Neil Robertson after the first session of the Tour Championship final in Llandudno as he closes in on a return to world number one.
The five-times world champion - who defeated Australia's 2010 world champion Robertson 10-4 to win the Players Championship at Preston's Guild Hall earlier this month - produced some immaculate break-building on his way to opening up a two-frame advantage in the best-of-25 frame final at a packed Venue Cymru.
A win for O'Sullivan will see him end Mark Selby's four-year reign as world number one, and return to the game's top spot for the first time since 2010. At the age of 43, he would be the oldest number one in the sport since six-times world champion Ray Reardon held it aged 51 in 1983.
Robertson pocketed the first two frames including a knock of 50 in the second frame, but failed to pot a ball in the next two frames as breaks of 74 and 97 - a quite sumptuous effort that was constructed with reds and blues with the pink and black out of commission - saw O'Sullivan restore parity at 2-2.
O'Sullivan - who made his 1000th career century in the final frame of his win over Robertson in Preston - was clearly in the groove with 71 and 67 enabling him to move 4-2 clear after the mid-session interval.
Robertson responded with a run of 54 in the seventh frame, but a few careless efforts late in the frame saw O'Sullivan pinch it on the black for a 5-2 lead.
The eighth frame was scrappy, but Robertson managed to produce a delightful double on the final yellow to put it beyond O'Sullivan, who opted against playing for snookers trailing by 30 points with only four colours remaining.
Robertson left the arena with a small punch of the air in relief, but O'Sullivan will be a heavy favourite to finish off the job with victory in the race to 13 frames and a £150,000 winner's cheque unless there is a dramatic change in form and fortune from both finalists.