Australian Open: Serena Williams Survives Wake-Up Call in Round 2

Top seed Serena Williams almost sleepwalked to an Australian Open upset Thursday before finding her mojo and bulldozing Vera Zvonareva 7-5, 6-0 to make it to the third round. (Ruthless Radwanska through to Round 3)

The 33-year-old American, chasing her sixth title at Melbourne Park, berated herself for a "passive" early effort when she had to save two set points against the Russian. (Venus Williams advances)

Williams, whose world number one ranking will be under threat is she does not win the tournament, did not rouse herself until the ninth game, when she was on the brink of going down a set. (Stan Wawrinka dodges Romanian rockets to advance)

"She started out really well, she was really aggressive and I was a little too passive," said Williams, who would take her total Grand Slam tally to 19 with another Australian title.

"Once I got down I thought 'Serena, you've done so well here you've got nothing to lose, just have fun and enjoy yourself' -- then I started to play a lot better."

Williams faces a tricky third round match against Spanish world number 24 Garbine Muguruza, 21, one of the new generation coming through in women's tennis.

But it was Zvonareva, who at 30 is one of the old guard, who was getting under the skin of the reigning US Open champion on Thursday.

The Russian, a former world number two who has slipped to 203 in the rankings after battling shoulder injury, signalled her intentions early when she went on the attack

The veteran, who lost to Williams in the 2010 Wimbledon final, showed no sign of succumbing to the fear factor the American holds over many of her rivals on tour.

She broke Williams in the third game and then did it again when the world number one broke back, forcing her opponent into the unaccustomed position of chasing the game.

Williams' frustration was evident when she hit a wild forehand into the crowd and bellowed with rage.

Zvonareva, a semi-finalist at Melbourne Park in 2009 and 2011, pressed on and brought up two set points with Williams serving at 5-4.

The American cannoned down consecutive aces to save the set, heeding the wake-up call and rattling off three quick games, including two breaks, to take the set.

Once she had the momentum, Williams looked unstoppable and powered through the second set without giving Zvonareva a chance.