French Open champion Gaston Gaudio reached the quarter-finals of the Stuttgart Cup after chalking up two contrasting wins on Thursday.

The second seed was made to fight all the way before securing a 3-6, 6-2, 7-6 win over qualifier Hugo Armando in his rain-delayed second-round match.

The Argentine was two points from defeat but eventually prevailed 7-5 in the tiebreak against the American before trouncing Czech Tomas Berdych 6-2, 6-4 in the third round.

"I think I was very lucky to win. The conditions are different from last week in Bastad. I was struggling yesterday against Hugo Armando, I wasn't feeling the ball at all," said Gaudio, who will next face Czech fifth seed Jiri Novak.

"Playing that third set this morning helped me get into the tournament. I am very happy with the way I played against Tomas Berdych this afternoon."

"My next match against Jiri Novak will be a tough one. I just about beat him at Roland Garros this year, I already know that I will have to fight it tomorrow."

Novak beat Russia's Mikhail Youzhny 7-6, 4-6, 6-1.

Gaudio's eighth-seeded compatriot Mariano Zabaleta, however, was beaten 7-5, 7-6 by 12th seed Albert Costa.

Spaniard Costa will face Nikolay Davydenko in the quarter-finals after the Russian overcame fourth-seeded Romanian Andrei Pavel 7-5, 1-6, 6-4.

Spain's David Ferrer ended the run of 18-year-old Frenchman Richard Gasquet 6-4, 6-2 and will next take on another teenager after Spain's Rafael Nadal upset third seed Juan Ignacio Chela 6-2, 6-3.