American swimmer Lynn Cox, braving bone-chilling 51-degree waters Friday, became the first endurance swimmer to cross a lengthy stretch of the world's deepest lake, swimming between two capes in Siberia's Lake Baikal.

Cox, 31, who also was the first swimmer to cross the Bering Strait, made the shore-to-shore 11.1-mile crossing between the two capes where the Angara River enters the lake in 51-degree water."Cox decided against attempting to cross (the entire) Lake Baikal, where the shortest distance is 25 miles and the water temperature was less than 53 degrees," Tass said.

Instead, "she swam for four hours and 20 minutes in cold waters, covering a distance of 18 kilometers (11.1 miles)," from Cape Tolsty to Listvyanka settlement on either side of the Angara riverhead.

She had moved the cold-water endurance attempt ahead by two days after the weather bureau predicted heavy storms over the weekend.

Cox, a native of Los Alamitos, Calif., said a leg cramp at the beginning of her swim was the most nerve-wracking part of her endeavor.

"I almost abandoned the attempt," she told Tass. "But I mustered my will and continued swimming."

The news agency said that at the beginning of the swim in the morning, nature was against her.

"She proved stronger than the elements," Tass said. "At some moment, she prevailed over them: the current eased, the wind subsided and the rising sun was illuminating her path. Her speed at times was over 4 kilometers per hour (2.5 mph)."

The challenge of Baikal is not distance, but the coldness of the waters, where even in summertime mid-lake water temperatures can drop below 50 degrees.

"At the halfway point, I felt stronger and the current had become more favorable," she said.

Her navigator, in a boat alongside, gave her apple juice and encouragement during her five rest stops.

She said she was afraid she had lost her bearing at one point. "Thank God this did not happen," she said.

Cox was greeted at the end of the ordeal by an orchestra improvised by American jazz musician Paul Winter, staying near Lake Baikal at the invitation of Soviet writer Valentin Rasputin.