Jessica Watson passed the Cape, a rocky headland near the point where the Atlantic and Indian Oceans meet, shortly after midnight GMT, marking the last landmark of her journey before she returns to Australia.
"I'm really thrilled to have another Cape down and to be in the Indian Ocean. Next target is Australia," Watson, 16, wrote on her blog.
Spokesman Andrew Fraser said the teen was in her highest spirits since leaving Sydney Harbour on October 18, and would cross the 15,000-nautical-mile mark of her 23,000-nautical-mile trip later this week, bearing directly for Western Australia.
She was travelling well ahead of schedule and on track to cross the finish line in Sydney before late May, one month ahead of deadline, he said.
"She's very happy, probably the best she's been during the trip," Fraser said. "It's great, we're all stoked, it's really encouraging."
Watson battled through a brutal, 12-hour storm in the South Atlantic last month, with hurricane-force winds and 10 metre (33 feet) waves pounding her bright pink yacht Ella's Pink Lady.
It was the first time the schoolgirl had experienced a "knockdown" -- when the mast goes below horizontal and dives into the sea -- and though the sloop suffered only minor damage, Watson was left bruised and shaken.
She is expecting relatively smooth sailing by comparison on her voyage back to Western Australia and then on to the east coast city of Sydney.
Watson has to complete the journey within eight months to break the record set by fellow Australian Jesse Martin, then aged 18, in 1999. Her supporters say the feat is the maritime equivalent of conquering Mount Everest.
When the teen left Australia on October 18, controversy raged over whether she was too young and inexperienced to undertake the challenge after she smashed into a massive coal freighter during a test sail in September.
Source: AFP Global Edition