Top-seed Casey beat Jerry Kelly 3 and 2. Els easily defeated Colin Montgomerie 6 and 5. U.S. Open champion Angel Cabrera had 11 birdies to beat Retief Goosen 6 and 5.
'I feel fortunate to be through to the next round,' Casey said. 'Jerry could have taken the match further or even won it but I did get fortunate and he missed a couple of putts.'
Padraig Harrington, Rory Sabbatini and Justin Rose were all upset. Anders Hansen beat Harrington 4 and 2, while Soren Hansen defeated Sabbatini 4 and 3 and Hunter Mahan downed Rose 6 and 5.
'There will be a lot of soul-searching as to why I'm playing well and couldn't produce it today,' said Harrington, the British Open champion who bogeyed three of the first five holes. 'Losing in the first round is very disappointing because I was thinking of going all the way.'
Two matches did not finish because of a two-hour fog delay in the morning.
Henrik Stenson and Woody Austin are all square after 17, and Andres Romero leads Niclas Fasth by two holes on the 15th. They will be completed Friday morning before the second round.
Els jumped out to a quick lead over Montgomerie.
'I drove the ball very, very well, as good as I've driven it in a while around Wentworth,' the South African said. 'We both played scrappy this afternoon, but it was good enough to get through.'
Montgomerie had little good to say about his round.
'I played rubbish, rubbish,' the Scotsman said. 'It was awful. I three-putted the first green and it set the tone for the day.
'I spent three days practicing my putting in the rain then I three-putt the first green. End of day. That was it. The worse I played, the better he became.'