Roger Federer was briefly rattled before reaching into the third round of the French Open with a 6-7 6-1 6-0 6-4 win over Spaniard Albert Montanes on Thursday.

The world number one, bidding for the only grand slam trophy missing from his collection, looked nonchalant in a scrappy first set before punishing his opponent with a string of winners.

The Swiss top seed will face Croatian Mario Ancic for a last-16 spot.

He also remained on course for an all Swiss quarter-final clash with Stanislas Wawrinka after the ninth seed breezed past Croatian Marin Cilic 7-6 7-6 6-1.

"The conditions were difficult, it rained a lot," said Federer afterwards.