Defending champion Lleyton Hewitt is struggling to come terms with his first-day Wimbledon loss to little-known qualifier Ivo Karlovic.

Hewitt became only the second defending champion in Wimbledon history and the first since the Open era began 35 years ago to lose in the first round, and the first top seed to lose so early.

But the world number two could not combat his 2.08m-tall opponent's big serve and power at the net, losing 1-6 7-6 (7-5) 6-3 6-4.

"I'll be disappointed for a while," Hewitt said.

"Every loss is different. I've had some Davis Cup losses that probably stick around fairly long. I'm sure this one's not going to go away by the end of the week."

It was a bleak opening day for Australian tennis, with Samantha Stosur, Evie Dominikovic and Nicole Pratt all losing.

But at least one player, South Australia's Paul Baccanello, triumphed over another Ivo, this time Swiss qualifier Ivo Heuberger, beating him 7-6 (10-8) 6-3 3-6 7-6(7-2) to set up a second-round match against Karlovic.

Karlovic, 24, of Croatia, was ranked 203rd in the world and playing his first grand slam match after 10 attempts to qualify when he pulled off his shock win, which will send Hewitt down to number four in the men's rankings.

He started nervously, unable to get a rhythm going, but found his feet on the grass during the second set and realised Hewitt was vulnerable.

"In the first set I was scared but after I thought I could beat him I started to play better," Karlovic said.

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