Hungary posted a first win in the men's water polo tournament to keep their dream of a record-breaking fourth consecutive gold medal alive, but senior players remained critical of the team's display.

The 17-15 victory over Romania was an important step towards the knock-out stages, but the team remain outside the top four qualifying spots in Group B, equal on points with the fourth-placed Romanians but behind on goal difference.

Hungarian goalkeeper Zoltan Szecsi, 34, who like three of his team mates is chasing a fourth Olympic gold medal, slammed the country's performance in the tournament so far after losses to both Serbia and Montenegro.

"We still don't have any confidence, that was the first step of gathering some. When we get back our confidence in the defence, we will be good," he said.

Five-times Olympian Tamas Kasas agreed that the team's defensive play had been "horrible" but said Hungary, the unmatched superpower in the sport with nine Olympic golds, should not be written off.

"I think we will advance. We will beat Great Britain and then I hope we will also beat the USA, and the Olympics will start at that point," said the 36-year-old who came out of retirement to play in the Games.

At the end of the third day of competition, the US team topped Group B, which features the best four teams from Beijing, after defeating Britain 13-7 to maintain their unbeaten run.

"After the first quarter we probably relaxed too much and they made our lives miserable, they really made us work for every goal," US centre forward Ryan Bailey, a four-times Olympian, said.

Gold medal contenders Serbia tied 11-11 against Montenegro, which was enough to put them into second spot in the group.

The Serbians failed to hold on to a two-goal lead in the final quarter, as Montenegro, cheered on by their prime minister Igor Luksic in the stands, rebounded in the last two minutes of what was a gruelling tussle.

On the other side of the tournament, Croatia stood top of Group A, after condemning world champions Italy to an 11-6 defeat - their first of tournament.

"It was just a lack of mental strength I guess. We knew it was going to be a hard game, and, it's the Olympics, everyone wants to kill everyone," Italy's Pietro Figlioli said.

Spain beat Australia 13-9 to go into second position, after a well-rounded display, which saw 10 out of the 11 Spanish outfield players on the scoresheet.

Greece, also in Group A, cruised past Kazakhstan 11-4 to post their first win of the competition.