The second-round drama in two hours, 13 minutes showed the Swiss just how tough it will be to hold off the field as he tries to extend his 235 weeks on top of the rankings.

With Rafael Nadal pressing, if Federer goes out in his title defence before the semifinals and the Spaniard wins the event - it would be his eighth of the season - the top ranking changes hands.

"I do enjoy the challenge that I'm the number one in the world," said the Swiss. "I'd rather it be me than rather it be him.

"I'm on a great run with my ranking. If I would lose it I would want it back. I hope I can get on a run for the two massive tournaments ahead of us (the Olympics and US Open)."

Andy Roddick, the 2003 champion, fell victim to the shoulder injury which he picked up in May, coming onto the court to announce that he could not play against German Philipp Kohlschreiber.

"I woke up with shoulder and neck pain," said the sixth seed. "I can hardly look from side to side. I wanted to come and give it a go. But I knew I wouldn't be able to finish so I wanted to give someone else the chance to play.

"It kills me that I have to walk off without playing," said Roddick.

Roddick was replaced in the draw by 252nd-ranked Jun Woong-sun of South Korea but the home fans had a show from Ginepri against Federer.

Federer beat Ginepri in their four previous matches, winning a break-marred opening set in a tiebreaker after grabbing a 1-5 lead in the decider.

Federer continued to have trouble in the second, losing serve in the 11th game for 5-6 and only just getting it back. Once again, the match needed a tie-breaker, with the Swiss surviving to force a third set.

Suddenly, as if switching on a light, the top seed ran away to victory, with 20 aces, 78 winners but conversions on only six of 16 break points.

"With this place there's just a little bit more danger around," said Federer of the fast Cincinnait courts. "I actually played OK throughout the match.

"I just missed some opportunities. I thought I was the better player actually throughout the match. I was down a set and 5-6, you know....

"It was dangerous today no doubt, but I believed in my chance all the way to the end."

If Nadal moved into the top position, the swoop would end a phenomenal run for Federer.

With his his four and a half years in the top spot, Federer remains fourth on the all-time list behind the Pete Sampras (286 weeks, 1993-2000), Ivan Lendl (270 weeks, 1983-1990) and Jimmy Connors (268 weeks, 1974-1983).