MELBOURNE, July 27 (Reuters) - There is an added pleasure for Pat Rafter in becoming world number one -- he knows it will annoy Peter Sampras.

The Australian, who took over the top ranking from French Open champion Andre Agassi on Monday, makes no secret of his rivalry with Sampras who has dominated men's tennis for much of the 1990s.

``There are times he says certain comments that annoy me. He just says some really funny things at the wrong times.

``We're out there busting our guts and he doesn't show a lot of respect at the end of the day...and he tries to play down the reason why he lost, and giving no respect to the other player.

``That's what really upsets me about him and the reason why I try to piss him off as much as I can.''

Double U.S. Open winner Rafter, speaking with reporters in a conference call from his Bermuda base, said: ``It's always nice to beat Pete, whether it's a ranking thing or in Davis Cup. I get an incredible amount of enjoyment from being on top of him and pretty well just annoying him.''

In the run-up to Rafter's successful U.S. Open title defence last year, Sampras was asked what the difference was between him and the Australian. ``About 10 grand slams,'' he replied.

The American leads their head-to-head clashes 8-4 but Rafter has won their last three matches, including the semifinal at last year's U.S. Open, a tournament Sampras considers his own.

Rafter said he was ``bloody happy'' to be the first Australian since John Newcombe in 1974 and only the 17th man to hold the men's number one ranking.

``It's a really exciting thing,'' said Rafter, who leads Wimbledon champion Sampras by just 79 points and is unlikely to keep the top spot for long under the tour's points system.

He said he was starting to believe anything was possible after his triumphs of the last two years.

``I don't feel the pressure... people want to beat you if you are two in the world or one in the world and it doesn't make any difference whatsoever to the way I approach things.''

``Now I set myself goals that I never would have really set myself realistically a while ago,'' said Rafter.

After leading Australia to victory in their Davis Cup quarterfinal against the United States in Boston, Rafter has spent the past week fine-tuning his game in Bermuda.

He faces a hectic few weeks defending valuable ranking points earned with victories last year at the Canadian Open and Cincinatti before the U.S. Open.

``I'm pretty solid going into Montreal (next week)...the workload's picking up each day,'' he said.