A third of the riders in Saturday's Olympic men's cycling road race did not finish and tennis officials said they were considering allowing heat breaks as Beijing's stifling humidity took a punishing toll on athletes.
Temperatures above 35Â°C, suffocating humidity and murky haze have combined to produce what competitors from several sports say are the most difficult conditions they have faced.
"It was a sauna out there," said Latvian cyclist Raivis Belohvosciks, one of the first of 53 riders to pull out on the gruelling 245km course that took the 143-strong road race field past Tiananmen Square to the Great Wall.
Some of cycling's biggest names were unable to handle the conditions, including Giro d'Italia winner Alberto Contador and pre-race favourite Stefan Schumacher.
Many riders said they had no trouble breathing at first, but struggled to recuperate from their efforts in the descents or on the easier sections of the circuit.
"I had a terrible headache. I don't know where it came from. Probably the pollution," said Germany's Schumacher, who showed impressive form on the Tour last month, winning two time-trials. "It feels like you're at 3,000 metres because of the air. You cannot breathe. The air is thick and there is smog."
The Olympic tennis tournament begins on Sunday and players preparing this week have found practice sessions a struggle.
"I've never, ever, played in worse humidity than this, these are extreme conditions," Serbian third seed Novak Djokovic told reporters. "It's hard to adapt but you can't cry and look for excuses."
In regular tournaments and grand slams, with the exception of the Australian Open, there is no provision for men to have heat breaks, but International Tennis Federation president Ricci Bitti said contingency plans were in place for the Olympics.
"It's within the power of our officiating team to take measures in case the situation is reaching an unacceptable level," he told reporters.
Track and field competitors have also been struggling to prepare for the athletics programme, which gets under way on 15 August, and are training in the morning or the evening.