TOKYO, July 24 (Reuters) - World champions the United States began their quest for a third straight Olympic gold medal in women"s water polo by storming into the record books with a 25-4 humbling of hosts Japan at the Tatsumi Water Polo Centre on Saturday. But the U.S. record for most goals scored in a single match at the Olympics stood just a few hours before being overhauled by reigning European champions Spain, who crushed South Africa 29-4 to lay down a marker of their own. Teenager Elena Ruiz, making her Olympic debut at the age of 16, top scored for Spain with five goals, while nine more of her team mates were also on target. Japan, who like South Africa are playing in their first Olympics, started brightly against the U.S. and even drew level at 3-3, but were outpowered and outclassed once their opponents settled into the match. "We got off to a rocky start, especially defensively," said U.S. captain Maggie Steffens, who scored five goals. "The Olympics gives you extra bit of energy and excitement and it was nice to see our team recover and take a deep breath." Stephania Haralabidis also scored five, while Madeline Musselman and Aria Fischer chipped in with four apiece for the all-conquering Americans, who have dominated women"s water polo in the past few years. Five other U.S. players got on the scoresheet as the match quickly descended into a drubbing. "We"re human, and we get nervous just like everyone else," U.S. coach Adam Krikorian said in response to a question on his team"s slow start. "It"s the first game of the Olympics and those jitters aren"t going to go away for us or for any other team. Sometimes it just gets us, but once we settled down, we were much better." Miku Koide scored twice for Japan, including her country"s first women"s water polo goal at the Olympics, with Yumi Arima and Eruna Ura also on target for the hosts. But U.S. goalkeeper Ashleigh Johnson was in scintillating form, saving 15 of the 19 shots she faced and shutting out the Japanese offence completely in the second and fourth quarters as her team made a dream start in Group B. Australia also started with a win, beating Canada 8-5 in Group A, with driver Bronte Halligan the pick of the Aussie players with three goals on her Olympic debut. The Russian Olympic Committee team, who won bronze in Rio five years ago, were locked in a fiercely physical battle with China in the day"s final match, but held on to win 18-17, with captain Ekaterina Prokofyeva helping her team snatch victory with two late goals.