RIO DE JANEIRO -- Tuesday will mark the start of the one-month countdown to the Rio 2016 Olympic Games as seven years of preparations reach their most critical juncture. We analyzes Rio's progress and identifies key challenges facing organizers ahead of the opening ceremony on August 5.

Workers are pictured at the construction site of the Rio 2016 Olympic Tennis venue at the Rio 2016 Olympic Park in Rio de Janeiro, Brazil, July 4, 2016. [Photo/Agencies]

Rio 2016 organizers have avoided many of the problems that beset the World Cup in Brazil two years ago, when most of the stadiums were running late amid huge budget overruns.

Last week city officials said that the construction of Rio's Olympic Park had reached 99% completion.

Five of the nine venues at the park are ready: Arena Carioca 1 (basketball), Arena Carioca 2 (judo, wrestling), Arena Carioca 3 (taekwondo, fencing), Future Arena (handball) and the aquatic center (swimming). Work on the athletes' village, media facilities and the Olympic Park hotel has also been completed.

Despite the encouraging progress at most Olympic facilities, not everything has gone to plan. Work is ongoing at the velodrome, a warmup pool at the Maria Lenk Aquatic center and two tennis courts.

City officials have said that the velodrome - the least advanced Rio 2016 venue - will be ready by mid-July, more than six months behind schedule.