Roger Federer survived a second-round scare at the Cincinnati Masters before fighting back to defeat Robby Ginepri.

The Swiss star, whose world No1 status is under threat from Rafael Nadal, was a set and 6-5 down to the unseeded American before sweeping through the deciding set to win 6-7, 7-6, 6-0.

The defending champion fired 21 aces but was still broken three times.

After his shock defeat by Gilles Simon in the second round at the Rogers Cup last week, Federer faced the possibility of losing his No1 ranking for the first time in 234 weeks if he fell to Ginepri, who is ranked 64th.

But if Federer does not reach the semi-finals, and French Open and Wimbledon champion Nadal wins the tournament, the Spaniard will take over top spot anyway.

Federer's lack of a killer instinct early on allowed Ginepri to break back to level at 5-5 immediately after losing his serve and the American went on to win a first-set tie-break.

Tied at 5-5 in the second set, Ginepri missed out on his first two break chances before cashing in on the third after an overrule by the chair umpire.

However, Federer broke back before taking the second tie-break.

He then made light work of the third set, dropping only one point on his serve.

Federer will next face 16th seed Ivo Karlovic, who defeated Argentina's Jose Acasuso 6-3, 7-6.

British No1 Andy Murray was in second-round action today against big-serving American Sam Querry.

Murray's brother Jamie and Swede Simon Aspelin secured a place in the third round of the doubles with a 6-3, 7-6 win over Czech pair Martin Damm and Pavel Vizner.