Big-serving American Andy Roddick [Images] eased past former world number one Juan Carlos Ferrero to make the semi-finals of the China Open on Friday, as a pair of unheralded challengers sent seeded opponents packing.

After struggling with his serve early, the former US Open [Images] champion smashed his racket in frustration on the way to losing the first set 2-6.

"That racket didn't want to be here any more, so I let it out ... I was pretty upset and pretty fed up with the way I was playing," Roddick told reporters.

The release seemed to help, however, as the 26-year-old quickly found his groove in the second set, eventually finishing with 23 aces in the 2-6, 6-3, 6-4 win over the unseeded Spaniard.

World number eight Roddick remains in the hunt for one of four remaining places for November's Tennis Masters Cup in Shanghai, which groups the year's eight top-performing players.

Swiss Roger Federer [Images], Spaniard Rafael Nadal [Images], Briton Andy Murray and Novak Djokovic [Images] of Serbia have already qualified.

Roddick faces a semi-final against world number 137 Bjorn Phau of Germany [Images], who upset third seed Fernando Gonzalez of Chile 6-3, 4-6, 6-3.

The 28-year-old Phau, whose biggest credits this year were two runner-up finishes at Challenge tour events, ran the world number 11 ragged.

"The match did not go as what I have expected ... He was so fast that I can only defend," Gonzalez said.

After eliminating top seed David Ferrer on Thursday, Israel's Dudi Sela played giant-killer again on Friday, upsetting sixth seed Tommy Robredo of Spain 6-4, 6-1 to sweep to his first ATP Tour semi-final at the $524,000 event.

The 23-year-old Sela overcame a nervous start to steamroll the world number 16 in an impressive display of controlled aggression and clean hitting.

Sela faces a semi-final clash with seventh seed Rainer Schuettler of Germany who upset French world number 12 Richard Gasquet 4-6, 6-3, 6-4.

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