Daredevil Nik Wallenda wowed the world with back-to-back walks on a tightrope between Chicago skyscrapers without a safety net or a harness, performing one of them blindfolded, as a crowd gasped and cheered.

"I feel incredible," Wallenda said at a news conference afterward on Sunday night.

He said strong winds and the steeper-than-expected angle of the first high wire caused him to hurry his performance. Wallenda had practiced at a 15-degree angle but said the wire was actually at 19 degrees.

"That cable looked like it was going straight up," he said.

Thousands of cheering fans packed the streets around the city's Marina City towers to watch the 35-year-old heir to the Flying Wallendas' family business.

It took Wallenda about six and a half minutes to walk the 138-meter stretch from the Marina City west tower to the top of a building on the other side of the river.

"I love Chicago, and Chicago definitely loves me," Wallenda said as he walked that wire, with the crowd below him screaming in support. "What an amazing roar!"

Wallenda undertook the next stage of his high-wire event blindfolded - a 28-meter walk between the two Marina City towers. He made the stretch in little more than a minute.

The Discovery Channel used a 10-second delay for the broadcast, which would have allowed producers to cut away if anything went wrong.

Journalists covering the event signed waivers relinquishing their right to claim emotional distress if they witness a catastrophe.

Two of his previous televised tightrope walks - over the brink of Niagara Falls in 2012 and across the Little Colorado River Gorge in 2013 - drew about 13 million viewers each.

Residents of Marina City were asked not to use laser pointers, camera flashes or drones that could interfere.

"It was amazing. I saw it with my own eyes," one spectator, Cynthia Garner, said afterward. "I was afraid when he first started, but once I saw that he didn't hesitate and just walked, I wasn't scared for him no more."