David Rudisha sealed a historic Olympic 800m double as Shaunae Miller's audacious dive and Thiago Braz's pole vault gold for Brazil lit up a wet Monday night in Rio.

Kenya's Rudisha was all class in a smooth 800m victory which made him the first man since New Zealand's Peter Snell in 1964 to win the event at consecutive Olympics.

But Rudisha was soon upstaged by the Bahamas' Miller, who threw herself head-first across the line to snatch victory from flummoxed American favourite Allyson Felix.

More was to come when Braz stunned French defending champion and world record-holder Renaud Lavillenie to win the pole vault, grabbing Brazil's second gold of the Games and bringing the sparse crowd to its feet.

Earlier, a wobble on the balance beam ended Simone Biles' bid for a record five gymnastics gold medals, and Rio had its latest safety scare when a giant suspended TV camera plunged to the ground, slightly injuring seven.

Miller remained lying on the track for some time after her unconventional dive, which followed a similar lunge across the line by Brazil's Joao Vitor de Oliveira to qualify from his 110m hurdles heats.

Her last-ditch plunge meant disappointment for America's Felix, who has Olympic gold medals over 200m, 4x100m and 4x400m but who switched to focus on the longer distance.

The desperate dives came on a treacherously wet night when runners clattered into hurdles and the discus slipped from hands before officials called a 20-minute suspension.

Conditions were so bad that an extra 110m hurdles heat was arranged for competitors who didn't qualify from the first two races, when the rain was torrential.

As the rain eased, Kenyan middle-distance star Rudisha emerged to stamp his class on the 800m.

The world champion and world record-holder hit the front in the final 300m and had one more gear than his rivals as he swept to the line in 1min 42.15sec.

In the pole vault, Lavillenie was the hot favourite but he was undone by Braz's Olympic record of 6.03m and had to settle for silver in front of the partisan crowd.

Gymnast Biles was confident of claiming her fourth Rio win and move into position for an unprecedented fifth on the last day of the competition on Tuesday.

But the tiny Texan wobbled badly on the balance beam when landing a forward somersault and had to put both hands down for support.