Andrey Moiseev's bid to become the first man in more than 50 years to win back-to-back Olympic Modern Pentathlon titles was on course after the Russian moved into first place with just two sports left.
Moiseev, 36 points adrift of China's Qian Zhenhua after the shooting and fencing sections of a competition that also features swimming, show-jumping and cross-country running was in front at the end of the 200 metres pool heats.
He led by 24 points from Qian and the Czech Republic's David Svoboda, who posted a new Olympic modern pentathlon record in the shooting.
In his fencing bouts against the other members of the 36 man field, Qian had 26 wins and nine defeats as did Moiseev.
Svoboda scored 191 out of a maximum 200 in the shooting, surpassing the previous Olympic best of 186 shared by the Belarus duo of Pavel Dougal (Sydney 2000) and Dmitry Meliakh (Athens 2004).
Moiseev is bidding to become the first man since Lars Hall of Sweden in 1952 and 1956 to win successive Olympic modern pentathlon titles.
Svoboda is also looking to make history by becoming the first man to win the overall title after winning the shooting phase.
Egypt's Amro el Geziry led in the pool with a new Olympic modern pentathlon swimming record of one minute 55.86 seconds, breaking Moiseev's mark in Athens four years ago of 1:58:88.
But El Geziry was still mid-way down the field in 17th place.
Moiseev turned in a time of 2:02:55 which converted into a total of 1,332 points, allowing him to overtake Svoboda (17th in the pool) and Qian (22nd).
Created by the French founder of the modern Olympics, Baron Pierre de Coubertin, the modern pentathlon is a demanding one-day event.
The initial section sees the 36 competitors given 20 shots from 10 metres with a pistol at a 155mm diameter target.
In fencing, they have a one-minute bout with every other competitor in the epee event, where hitting any part of the body counts.
The show-jumping is a 400m course with 12 fences, although the toughest part is that the athletes are on horses they've never ridden before, their mounts chosen by lottery.
Then the final 3,000m run is staggered with the leader starting first - a three-point lead is a three-second advantage - meaning the first to cross the finishing line is the winner of the overall competition.