Melbourne, Jan 22 (IANS): Top seeds continued the trend of comeback wins as Serena Williams, men's defending champion Stan Wawrinka and Asia's top ranked male, Kei Nishikori, showed vulnerability in the second round of the Australian Open here Thursday.

A day after Rafael Nadal, Roger Federer and Maria Sharapova were all forced to fight back from a set down, Kei was far from his best against Croatia's Ivan Dodig, who took the first set and only narrowly missed the second, reports Xinhua.

World No.1, Novak Djokovic had no problems, however, mercilessly bulldozing Russian Andrey Kuznetsov 6-0, 6-1, 6-4.

Nishikori, who eventually found form to win 4-6, 7-5, 6-2, 7-6 (7-0), said he was not yet playing his best tennis.

"I think I'm getting close," said Nishikori. "Maybe these couple of matches weren't played at 100 percent but I'm still winning in three or four sets."

"I think it's getting there."

The highest ever ranked player from Asia will next play the American Steve Johnson, who knocked out 30th seed Santiago Giraldo 6-3, 6-4, 6-2.

Five-time Australian Open women's champion Serena Williams was forced to save three set points before she took the first set and ran away with the match to progress with a 7-5, 6-2 win over Vera Zvonareva.

Zvonrareva, a former World No.1 who lost to Williams in the 2010 Wimbledon final, was on the verge of breaking the current top ranked player for the second time at 3-5, only to lose the next 10 games.

"Things really clicked," said Williams. "I had no option but for things to click."

Serena's sister, Venus, also got through to Saturday's third round with a straight-sets win over compatriot Lauren Davis.

"She's really motivating," said Serena.

"It makes me look behind my shoulders and I have to play better, I want to do better."

The defending men's champion, World No.4 Stan Wawrinka faced a 242 km/hr serve from Marius Copil and struggled to get a racquet on those marginally slower.

Wawrinka, however, prevailed in each of the match's two tie-breaks before breaking the fast, if sporadic, serve of Copil to win 7-6 (7-4), 7-6 (7-4), 6-3.

"He was playing great. He was serving big and putting lots of pressure on," said the Swiss player.

"I wasn't playing my best tennis but I'm happy with the way I fought today and the way I did win the match."

Frenchman Adrian Mannarino almost pulled off a big upset against 12th seed Feliciano Lopez after leading two sets to love and 4-0 in the third.

Mannarino was overcome by heat stress as the temperature pushed beyond 32 degrees celcius and, after looking on the verge of fainting over the final 20 minutes, he retired with the score at 4-6, 4-6, 7-6 (7-3), 4-0.

However, the performance of the day remained with women's title hopeful Agnieszka Radwanska who was off court before midday.

The 2014 Australian Open semi-finalist's emphatic 6-0, 6-1 win over Sweden's Johanna Larsson took a mere 44 minutes.