TOKYO, Sept 21 (Reuters) - The United States grasped their second chance to progress to the final of the men's 4x400 metres relay after comfortably beating Kenya in a slightly surreal winner-takes-all run-off at the World Athletics Championships on Sunday.

South Africa were unable to do the same as they missed their target in their against-the-clock rerun in the 4x100 metres.

The U.S. and Kenya were given the opportunity to race for an additional place in the final later on Sunday after both were judged to have been impeded at a changeover as they failed to qualify from Saturday's heats.

In the strange atmosphere, with a scattered crowd watching the morning events of the decathlon, the teams ran in their original lanes - the U.S. in three and Kenya in seven.

Despite the quick turnaround and early start, the U.S. looked far more impressive than on Saturday, when they were already struggling near the back even before the obstruction.

After a tight first lap, Demarius Smith pulled clear on the second leg, Bryce Deadmon maintained the lead and Jenoah McKiver brought them home in 2:58.58 - their fastest time of the year, albeit in a rarely-run event. Kenya clocked 3:00.39.

The US have won nine of the last 10 world 4x400s and 12 of the 19 run and have never missed a final. However, they will be allowed to change only one runner for Sunday night's final, piling on the pressure to keep up their run.

In the 4x100m relay, South Africa had been impeded by the Italian team on Saturday and were given the chance to run a solo time trial, using the same four athletes in the same lane.

Their target was to match eighth-placed France's time of 38.34 to proceed as a ninth team in the final, but they just missed out with 38.64. In keeping with the chaos surrounding the relays, the stadium clock did not initially stop when Akani Simbine crossed the line, adding even more tension, before the time eventually appeared on the scoreboard.

All four relay finals take place later on Sunday, the final day of the championships.

Reporting by Mitch Phillips; Editing by William Mallard

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US through to 4x400 final, South Africa miss out on 4x100