DUSSELDORF, May 22: It was the day of the underdog at the 1.85 million euro (2.36 million dollar) ATP World Team Championship with favourites the United States and Argentina both falling to opening group defeats on Monday.

Gaston Gaudio, French Open champion in 2004, and World No. 3 David Nalbandian spearheaded a strong Argentina challenge but the Czech Republic showed no fear to win the red group match.

Gaudio started as strong favourite against Czech Robin Vik - ranked 57 places below him in the world rankings - but was convincingly beaten in straight sets.

The defeat piled the pressure on Nalbandian, a winner at the recent ATP event in Estoril, to win his singles match against Tomas Berdych to keep Argentine hopes alive.

However Berdych, ranked 20th in the world, triumphed 6-4, 6-4 to seal a Czech win and render the doubles match irrelevant.

Argentina will now have to win their last two matches against Germany and Italy to have a chance of reaching Saturday's final.

The United States also started the tournament as one of the favourites for the title but came unstuck in their first match against Davis Cup holders Croatia.

Croat Ivan Ljubicic and American James Blake, ranked fourth and eighth in the world respectively, clashed in a high-quality first match.

Blake took the opening set 7-5 but Ljubicic levelled with a 6-3 triumph in the second set.

The 26-year-old Ljubicic triumphed on a tie-break in the third set to overcome Blake.

America's World No. 5 Andy Roddick was missing due to a shoulder injury so Robby Ginepri took his place against Ivo Karlovic in the singles match.

Ginepri proved a more than adequate stand-in recording a comfortable straight sets win meaning the doubles match would be decisive.

America looked favourites with Mike Bryan and Bob Bryan - the top doubles pair in the world - to call upon but it was the in-form Ljubicic and Karlovic who won in straight sets to seal a historic win.

Results:

Red Group

Czech Republic 2 Argentina 1

Singles

Robin Vik (CZE) bt Gaston Gaudio (ARG) 7-6 (7/5), 6-3

Tomas Berdych (CZE) bt David Nalbandian (ARG) 6-4, 6-4

Doubles

Jose Acasuso/Sebastian Prieto (ARG) bt Tomas Berdych/Leos Friedl (CZE) 6-0, 2-6, 10-3 (match tie-break)

Blue Group

Croatia 2 United States 1

Singles

Ivan Ljubicic (CRO) bt James Blake (USA) 5-7, 6-3, 7-6 (7/4)

Robby Ginepri bt Ivo Karlovic (CRO) 6-4, 7-6 (7/1)

Doubles

Ivan Ljubicic/Ivo Karlovic (CRO) bt Mike Bryan/Bob Bryan (USA) 6-4, 7-6.—Agencies