Luke Donald tees off at Wentworth on Thursday in the knowledge that a successful defence of the BMW PGA Championship would take him back to the world No.1 ranking.

Donald took top spot for the first time when he beat Lee Westwood, who himself held the No.1 ranking at the time, in a play-off a year ago.

He stayed there until Rory McIlroy's victory at the Honda Classic in Florida in early March, and there have been four further changes between the pair since.

The 23-year-old Northern Irishman, who defends his US Open crown in San Francisco next month, is also in the field for the European Tour's flagship event.

If Donald finishes second, McIlroy would have to win to keep hold of the No.1 position and, if McIlroy misses the halfway cut – he did so on his last outing at the Players Championship two weeks ago – Donald would need to finish eighth or better. Westwood is presently third in the rankings, but just too far back to have a chance of leaping over the other two this week.

Mike Weir, the former Masters champion from Canada, is a late withdrawal. He injured his lower back in the first round of the HP Byron Nelson in Texas last Thursday.

Danny Denison, a Challenge Tour graduate from Leeds, comes into the field and will be making his debut in the championship five years after breaking both legs in a car crash.

The 27-year-old spent nearly two years on the sidelines before resuming his career. He was joint runner-up in the St-Omer Open last year and won a Challenge Tour event in Denmark. He has made the halfway cut in only two of his 10 starts so far this season.

n Graeme Brown, from Montrose Links, stole a march on his rivals in the £3370 Deer Park Young Professionals' 36-hole tournament at Livingston, writes Colin Farquharson.

The 32-year-old Scotland amateur internationalist in 2004 – before that he played on the US college circuit – hit the ground running with a four-under-par 68 that gives him a two-shot lead at the halfway point.

His nearest rival is 23-year-old Neil Fenwick, of Dunbar, winner of the Young Professionals' Order of Merit last year.

Sharing third place on 72 are David Laing (Craigielaw) – he had an eagle 3 at the eighth and a double-bogey 7 at the 15th – and Nairn Dunbar's Malcolm Isaacs. Emma Fairnie (Gullane), the only girl in a big field, was a creditable joint 29th on 78.

*A second win in three starts on the PGA Tour for the American Jason Dufner has lifted him to a career-high official world ranking of No.14. Dufner beat his compatriot Dicky Pride by a stroke at the Byron Nelson Classic.

Nicolas Colsaerts, of Belgium, climbed from 51st to 32nd in the rankings after his victory over Graeme McDowell in the final of the Volvo World Matchplay Championship in Spain. Paul Lawrie, beaten by Colsaerts in the semi-final, rose one place to No.40 and is now just nine places behind the leading Scot, Martin Laird who remains at No.31 after a free week.

Richie Ramsay and Stephen Gallacher remain just inside the top 100 and Scott Jamieson is the only other Scot in the top 200.