Grigor Dimitrov celebrates after beating Mackenzie McDonald in the second round of the Australin Open on Wednesday. Photo: AP Photo/Andy Brownbill)

MELBOURNE - Relieved world No.3 Grigor Dimitrov survived a massive scare to beat American qualifier MacKenzie McDonald in five sets and reach the third round of the Australian Open on Wednesday.

The Bulgarian needed to pull out all stops to see off the 186th-ranked McDonald, 4-6, 6-2, 6-4, 0-6, 8-6 in the 3hr 25min night match on Rod Laver Arena.

Dimitrov, who is ranked only below Rafael Nadal and Roger Federer, drew on all his experience to avoid his first defeat to a qualifier at a Grand Slam.

McDonald attacked the third seed's signature backhand, limiting him to only five winners in the marathon match and restricted Dimitrov to just 28 percent of second service points with the Bulgarian committing nine double faults.

It was a mighty relieved Dimitrov who lived to fight another day, and line up a third round clash with young Russian Andrey Rublev.

"What can I say, it was a tough day," he said. "I'm most happy with that. He was playing absolutely unbelievable, he deserves a lot of credit."

Dimitrov said the pair had practised together in California, where McDonald had a successful college tennis career with UCLA.

"He has improved so much. I just had to fight and the experience in the end really helped me," Dimitrov said.

"I'm very happy with win, not because of the way I played, but because the way I fought. It all came down to a few points here and there."

The stylish Bulgarian claimed the biggest title of his career when he beat Belgium's David Goffin in the ATP Finals in London among his four titles for the year.

But he has never been beyond the semi-finals of a Grand Slam, reaching the last four at Wimbledon in 2014 and in Melbourne last year.

McDonald, 22, was making only his second appearance at a Grand Slam after his debut at the 2016 US Open.

The American has a successful college tennis background, winning the 2016 NCAA singles and doubles titles before turning professional.

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