Dubai: Former world No 1 Roger Federer has paid tribute to the chasing pack of players for closing the gap on the top four.

Federer, Novak Djokovic, Rafael Nadal and Andy Murray have been at the top of the men's rankings for a long time and have dominated the Grand Slams, winning all but one of the four big events since 2005.

But the Swiss legend believes the likes of Juan Martin del Potro, Jo-Wilfried Tsonga, David Ferrer and Tomas Berdych are steadily closing the gap.

"I think obviously you look at the rankings and in points the top four guys are ahead. But right behind us there is a very good, solid, tough group with Ferrer, Berdych, Jo-Wilfried Tsonga and now Del Potro," Federer said after edging past Del Potro thanks to two tie-break victories in the Dubai Duty Free Men's Open semi-finals on Friday.

"They get great respect from all of us. I had a sort of, I don't want to say a losing record, but I lost to Jo [Tsonga] a couple times and I lost to Berdych a couple times. Next thing you know, you're losing more against these guys than you're winning. You're looking at maybe dropping outside of the top eight if that trend continues."

Praise for Del Potro

Friday's meeting between Federer and Del Potro was their third this year, with the Swiss ace winning all three. But the scores at each match indicate that the Argentinian — who beat Federer to claim the 2009 US Open final — is fast catching up.

"This is where Juan Martin is going to be now — I guess seeded in the top 8 maybe by the French Open, I would assume. That's going to be a big help for him just in terms of trying to avoid the big seeds before the quarter-finals. That's what he did not have last year. That's maybe also one of the reasons that he couldn't get his ranking up higher, potentially," he said.

"So I believe he can be up there very quickly. I mean, he's had a tough week coming directly from Marseilles [where he won the title] and backing it up after the finals in Rotterdam, this is tough stuff. I'm impressed by his play right now."