New York, Apr 20 (IANS): Former World No.1 tennis player Andy Roddick heaped praise on 20-time Grand Slam champion Roger Federer, especially for his unique serve technique which according to him is 'frustrating' for his opponents.

Roddick, who is regarded as one of the finest servers of the game, stated that Federer's serve is a combination of former greats Pete Sampras and Andre Agassi.

Roddick feels that Federer has the ability to alter the pace of his serve like Agassi and he also possesses the pinpoint accuracy of Sampras, which makes it unpredictable for the opponents.

"Andre Agassi was a player who messed around with his ball toss and the pacing of his serve. Pete Sampras was a more powerful server and had very good placement. I feel that Roger Federer's serve is a hybrid of Pete and Andre," Roddick was quoted as saying by Tennis World USA.

"Roger had the incredible ability to throw any kind of serve at any time. He could throw 97 mph kick serve one time then throw a very powerful serve out wide. He was never predictable and that was what was so frustrating."

Roddick has immense experience of playing against Federer as the duo clashed 24 times on ATP circuit with the Swiss holding an impressive 21-3 record over his American counterpart.

Federer has also got the better of Roddick in four Slam finals -- thrice at Wimbledon (2004, 2005, 2009) and once at US Open (2006).