Novak Djokovic was forced to retire at the end of the third set against Nikolay Davydenko as Serbia lost 3-2 to Russia in their Davis Cup World Group tie in Moscow.

The recently-crowned Australian Open champion was struggling with flu-like symptoms and called time having failed to win the third set, complaining of dizziness and general exhaustion to give Russia a 3-1 lead.

He went straight from the Luzhniki arena to his hotel and from there to a local hospital for treatment.

Viktor Troicki followed up his five-set defeat against Davydenko with victory over Dmitry Tursunov in the dead fifth rubber, sealing the match 7-6 (9/7) 4-6 6-3.

Russia now face a home quarter-final against the Czech Republic after they overcame Belgium.

The Czechs secured their passage to the last eight with a 3-2 triumph although Steve Darcis and Ruben Bemelmans clawed two points back for the Belgians.

Darcis defeated Lukas Dlouhy 7-6 (7/1) 6-4, while 20-year-old Bemelmans earned his first Davis Cup win after Czech doubles specialist Pavel Vizner retired with the score at 6-7 (4/7) 7-5 2-2.

Twins Mike and Bob Bryan split a pair of singles matches to wrap up a 4-1 first-round victory for the United States over Austria.

The Bryans helped the defending champions book their place in the quarter-finals when the doubles pair secured a 6-1 6-4 6-2 win over Jurgen Melzer and Julian Knowle on Saturday.

The Americans, who opened on Friday with singles victories by Andy Roddick and James Blake, will now face France in an April quarter-final.

It will be the 15th meeting between the countries and the French beat the Americans 3-2 in their last encounter, a semi-final clash in Paris back in 2002.

After the Bryans had clinched the tie on Saturday, US captain Patrick McEnroe decided to rest Roddick and Blake for Sunday's dead rubbers.

Mike Bryan lost to Stefan Koubek after retiring in the second set with an injured left hamstring. Koubek was ahead 7-5 1-0 when the American called it a day.

Bob Bryan then posted a 6-0 3-6 7-6 (7/3) victory over Werner Eschauer to complete the best-of-five tie.

France rounded off a 5-0 sweep of Romania as Michael Llodra and Arnaud Clement won their singles matches.

Llodra posted a 7-6 (7/5) 7-6 (9/7) victory over Andrei Pavel, while Clement defeated Horia Tecau 7-6 (7/3) 2-6 6-4.

Elsewhere, veteran Jonas Bjorkman beat Harel Levy in four sets to give Sweden a 3-2 win over Israel.

The 35-year-old fought back after losing the first set to win 0-6 6-4 6-3 7-6 (8/6).

Thomas Johansson had previously levelled the tie at 2-2 when he beat Israeli number one Dudi Sela 7-6 (8/6) 6-1 7-5.

Sweden will play Argentina in the quarter-finals after they completed a comfortable 4-1 victory over Great Britain.

Philipp Kohlschreiber gave Germany a 3-2 win over South Korea after beating Lee Hyung-Taik 6-0 4-6 6-1 7-6 (7/1).

Jun Woong-Sun won the final match, defeating Michael Berrer 6-1 3-6 6-4.

Germany will next face Spain, who defeated Peru 5-0.

Tommy Robredo beat Mauricio Echazu 6-4 6-1, while Nicolas Almagro saw off Ivan Miranda 6-2 6-3 to complete the whitewash after the Spaniards had clinched the tie on Saturday.