Rafael Nadal's Spain eased into the quarter-finals at the Davis Cup finals, beating a weakened Croatia to send the defending champions out of the event.

With Croatia missing Marin Cilic and Borna Coric, doubles specialist Nikola Mektic had to play singles against Roberto Bautista Agut, losing 6-1 6-3.

Nadal then beat Borna Gojo 6-4 6-3.

Meanwhile, Serbia, who won the Davis Cup in 2010, put themselves in pole position in Group A with a 3-0 victory over Japan at the Madrid event.

Filip Krajinovic defeated Yuichi Sugita 6-2 6-4, Novak Djokovic thrashed Yoshihito Nishioka 6-1 6-2, and Janko Tipsarevic and Viktor Troicki clinched a tight doubles match against Ben McLachlan and Yasutaka Uchiyama, winning 7-6 (7-5) 7-6 (7-4).

Germany also triumphed 3-0 as they defeated Argentina. Philipp Kohlschreiber battled back from a set down to defeat Guido Pella 1-6 6-3 6-4 and Jan-Lennard Struff was too good for Diego Schwartzman with a 6-3 7-6 (10-8) success.

In the doubles, the German pairing of Kevin Krawietz and Andreas Mies overcame Maximo Gonzalez and Leonardo Maye after the longest tie-break in Davis Cup history as they won 6-7 (4-7) 7-6 (7-2) 7-6 (20-18).

Great Britain's near nine-hour tie with the Netherlands,which ended in a 2-1 victory for GB, meant that the tie between Australia and Belgium to decide who will win Group D started two hours later than scheduled.

Spain's victory over Russia on Tuesday had finished in the early hours of Wednesday and a similarly late finish looked on the cards for both Australia v Belgium and the Group F match between the United States and Italy as questions over scheduling arose at the re-vamped competition.

The United States were at the centre of controversy after group winners Canada conceded their doubles rubber on Tuesday when they had an unassailable 2-0 lead.

It meant the US were given a 6-0 6-0 win for the doubles, which could potentially help them qualify for the last eight as one of the two best runners-up.

Organisers the ITF said three Canadian players had been declared unfit to play, but Britain's Andy Murray was among critics of the decision to forfeit, saying: "I don't think that's good.

"So where Canada may have felt that that was a dead rubber in theory for them because they were already through, that could have implications to all of the teams potentially that might finish in second place."