David Rudisha retained his 800m title with an imperious display on another night of drama in the Olympic Stadium in Rio.
The Kenyan, who broke the world record in taking gold at London 2012, hit the front with 300m to go and stayed strong to come home in one minute 42.15 seconds.
There was a surprise in the women's 400m, where Shaunae Miller edged out Allyson Felix to claim victory, while home favourite Thiago Braz da Silva won a thrilling pole vault competition that was completed just before midnight local time.
A torrential downpour at the beginning of the evening threatened to put a dampener on the fun, with athletes forced to take shelter under the stadium as puddles formed on the track.
As a result, competitors who failed to qualify in the opening two rounds of the 110m hurdles - those who were out in the worst of the weather - were given another opportunity to advance as fastest losers, with the pole vault taking almost three and a half hours to complete as a result of delays caused by the downpour.
Rudisha was the first to secure gold on Monday night once conditions had improved and recorded the fastest time in the world since his unforgettable run in London.
Algeria's Taoufik Makhloufi claimed silver in 1min 42.61, with American Clayton Murphy securing bronze.
Rudisha's 1:40.91 world record from four years ago was achieved with an awesome display of front-running and although injuries have prevented the 27-year-old from reaching that level since, the two-time world champion still remained in total control.
His compatriot Alfred Kipketer set an extremely fast early pace, going through 400m in 49.23 seconds, but Rudisha sped past him down the back straight as he showed his experience of competing under pressure.
In the other medal event on the track on Monday, Miller denied Felix a fifth Olympic gold after she dived across the finish line.
The 22-year-old Bahamian went out hard from lane seven and led coming into the home straight, although Felix looked set to reel her in.
As the world champion tied up, though, Miller launched herself over the line, either by accident or design, to take victory in a new personal best of 49.44s.
Felix had to settle for silver in 49.51, with Jamaica's Shericka Jackson taking bronze.
American Felix had hoped to bid for a 200m and 400m double in Rio, but she failed to qualify for the United States team over the shorter distance after struggling with an ankle injury earlier in the season.
And her bid for a first Olympic 400m crown, after gold in the 200m, 4x100m and twice in the 4x400m, also fell short.
Arguably the biggest drama of the night in the Olympic Stadium was saved for the end of the night, however, as unfancied Da Silva set a new Olympic record of 6.03m to see off the challenge of defending champion Renaud Lavillenie.
The Frenchman looked on course to win another gold in the pole vault but, having set up a head-to-head as other competitors fell by the wayside, Da Silva pulled out the jump of his life.
Lavillenie appeared visibly stunned and with a partisan home crowd against him, he failed to clear 6.08m having already recorded two failures at 6.03m to hand gold to the Brazilian.