John Cook took advantage of Fred Couples' self-destruction to win the $2.5m Byron Nelson Classic by three strokes in Dallas yesterday.

After having started the final round four strokes behind Couples, Cook shot a closing five-under-par 65 on the Las Colinas TPC at the Four Seasons Resort. However, as well as the Californian played, he was helped in no small measure by Couples, who was tied for the lead until he triple-bogeyed the par-three seventeenth after his No.6-iron tee shot bounced off a rock into the lake fronting the green.

Cook, playing in the pair ahead, had already made a safe par, and another par at the last gave him a 15-under 265 total and his tenth US Tour victory.

''What happened to Fred was unfortunate but sometimes you need these things to happen to win,'' said Cook, who collected $450,000, the biggest prize of his career.

He has blown a few tournaments himself, most particularly the 1992 Open at Muirfield, where he missed a short putt at the seventy-first hole before bogeying the last to lose to Nick Faldo by one stroke.

Couples, who shot 72 to tie for second with Hal Sutton and Harrison Frazar at 12-under 268, could not believe his shot went into the water on 17. ''When it was in the air I thought it was going to be really good. Then it started drifting. When it hit the rock I was shocked,'' he said.

Ernie Els will need to improve his putting to have a chance of winning this week's Volvo PGA Championship at Wentworth.

Els, returning from a three-week break, struggled on the greens here to finish at even-par 280. ''I never had my putting stroke from day one,'' Els said after a closing 71. ''The rest of my game is not too bad. I hit the ball quite nicely. I've just got to work on my putting technique and get that sorted out.''

On a brighter note, South African Els regained the world No.1 ranking. He relinquished it to Tiger Woods a week ago when the American won the BellSouth Atlanta Classic, but Woods needed to finish no worse than second yesterday to stay number one.

However, Woods (67) only tied for twelfth at eight-under 272: ''Last week I had it going for one day,'' he said.

''This week I hit the ball good in spurts and putted good in spurts. I couldn't quite put them together.

''It could have been better, but it could have been a lot worse. It was just one of those weeks where I didn't get anything going at the same time.'' Final scores (USA unless stated):

265 - J Cook 66, 68, 66, 65.

268 - H Sutton 66, 65, 68, 69; F Couples 66, 67, 63, 72; H Frazar 64, 68, 66, 70.

269 - S Stricker 67, 72, 65, 65.

270 - S McCarron 66, 72, 68, 64; P Mickelson 66, 68, 69, 67; B Friend 63, 70, 68, 69.

Other scores included:

272 - T Woods 65, 71, 69, 67.

273 - M O'Meara 67, 69, 69, 68.

276 - J Leonard 68, 69, 69, 70.

277 - T Watson 64, 70, 72, 71.

280 - E Els (S Africa) 69, 69, 71, 71.