London - Roger Federer was pausing only briefly to delight in his return to year-end world number one status a year after losing the honour to Rafael Nadal in Shanghai. There was the more pressing matter to hand of Thursday's grudge duel with Argentine Juan Del Potro as both men battled to reach the weekend semi-finals of the season-ending ATP World Tour Finals.
"Obviously, that was one of my big goals for this season," Federer said after winning top ranking glory for the fifth time in six years through his group defeat of Andy Murray in three sets Wednesday.
"It's one of the greatest performances I think I have achieved. With what also happened off the court: getting married, having twin girls, reaching all four major finals ... playing so well at the most important moments, it's been the key this year."
Federer was most pleased to have become the only man besides Ivan Lendl to return to the top ranking at season's end after once losing it.
With five year-end number one finishes, Federer ties the mark of Jimmy Connors and now stands just one back of Pete Sampras on the list of all-time champions. Sampras finished as the top player six straight years, from 1993-98.
"I had a rough 2008, coming back this year and being able to dominate and play at the top when the depth in tennis is so great at the moment. I think it's a wonderful achievement. Of course, it's official. It's a wonderful feeling," said Federer.
But top of Federer's immediate agenda is his meeting with Del Potro, who beat him in five sets to steal the US Open crown two months ago from the record 15-time Grand Slam champion.
"Last time we played, it was a fantastic match in New York," said the Swiss.
"And it seems like he's playing well, whereas in the beginning I thought he might have some issues maybe with his wrist or with his abdominal strain.
"But he seems fine now. So he looks like he's getting better as the tournament goes on," he said of an opponent whose form slumped over recent weeks as injuries and realisation of what he had accomplished finally sunk in.
Places for the weekend last four in London are still wide open in both round-robin groups, with possibilities to be narrowed during Wednesday play in Group B as Novak Djokovic faced Robin Soderling and Rafael Nadal took on Nikolay Davydenko.
"All groups are very even," said Federer. "I think we have only had one straight-set match so far (Soderling-Nadal). Other than that, it's always gone three sets and been very hard fought."
