The European Tour Order of Merit race is set to go right to the wire at Valderrama tomorrow - and Paul Casey, Padraig Harrington, David Howell and Robert Karlsson all have a chance to end the season as number one.

It could well be the most exciting end-of-season tussle since Colin Montgomerie pipped Sam Torrance in 1995 with a three-foot par putt on the final green of the final event.

With one round left in the Volvo Masters money leader Casey remains in the driving seat, but only after Howell and Harrington failed to make the most of charges through the field.

Howell, who on Wednesday looked on the point of pulling out of the event because of a shoulder injury, has to finish in the top two and when he had five successive birdies early in his third round he was joint leader with Swede Henrik Stenson.

However, he three-putted the next, then covered the back nine in a disappointing 39 and at level par was back down in a tie for eighth place on level par, three behind overnight leader Jeev Milkha Singh from India.

Harrington, for whom a top three finish could be good enough with Casey not in the top 30 after his 70, moved into a tie for second spot when he turned in 33.

But then came three successive bogeys, two of them the result of three-putting, and that put the Dubliner on one over after his 72.

And Karlsson, for whom first or second is required, was on level par with Howell after he mixed five birdies with four bogeys.

"I felt in control throughout the round and then I take three blips in a row," said Harrington referring to the run of bogeys on the back nine. "Normally I'd think of playing those three holes in two under but instead I played them in three over."

"But that's the nature of golf and you need things to go for you in order to win a golf tournament. Hopefully things will be different tomorrow.

"The bogeys definitely knocked some momentum out of me but I made a good par at the last which I was pleased with," added Harrington, who feels he still has a slim chance of winning.

"To be four behind is not the end of the world but there are a lot good players between me and the leader. Tomorrow I will be going out there to win the tournament. I know second place would be good enough but I can't think like that, I have to go out there to try and win."

Should Harrington manage to secure second place it would be the 30th runner-up finish of his career. However, you get the impression that, on this occasion, he wouldn't mind too much.

Lee Westwood (72) was three clear in the tournament when he played the first six in two under against the three over of playing partner Stenson. But Westwood had a hat-trick of bogeys around the turn and slipped into a tie for second on two under, alongside Swedish pair Johan Edfors (69) and Henrik Stenson (73) plus Sergio Garcia (70).

Casey, who had been joint 35th at halfway, said of his round: "I thought I would move up a bit more. I didn't think it was easy and I was happy with that.

"I've not really crunched numbers, but I know I can maybe make Padraig finish second rather than third.

"Unfortunately the tournament is lost. There's nothing I can do about that. The first two days (when he was struggling with food poisoning) pretty much killed off my chances of being in contention.

"Whatever happens, happens. It would certainly be disappointing not to finish top but I don't think it would be a crushing disappointment. If it does not happen I have plenty more chances. But I certainly want the Order of Merit before I hang up the clubs."

Howell said: "It's exciting and I'm looking forward to it. I've given myself a chance and I definitely would have taken that on Thursday morning.

"Golf is such a game of momentum. I was in a nice little zone early on, but then came the three-putt. Classic error, momentum lost.

"After being five under through seven it was obviously disappointing only to score 70, but would I have taken it before the start? Possibly."

Defending champion Paul McGinley moved up into a share of 19th after today's level par round and that left the Dubliner on two over and not without an outside chance tomorrow.

Graeme McDowell continued to slide down the leaderboard - following his opeing 67 for the first round lead - and today the Ulsterman shot 74 to find himself on six over.

Damien McGrane finds himself at the wrong end of the elite 60-man field as his 74 left the Meath man 13 over.