Reuters/Brisbane

The number one seed won in 98 minutes to reach the quarter-finals but her 36 unforced errors and 55 percent serving accuracy showed how much Williams has to work on with the first grand slam of 2014 less than a fortnight away.

German Petkovic was exactly what the defending champion needed at this point in the season. Ranked 43rd in the world, but a top-10 player before injuries hampered her 2012 campaign, she ensured the five-times Australian Open winner had to fight for every point.

“It was an intense match, which was really good for me,” Williams told reporters. “She’s a good player. She’s been in the top 10 before and she was on her way to being even better.”

In their only previous match Petkovic took the American to three sets, on clay in Rome in 2010, and looked capable of doing so again after nullifying an early break and taking a 4-3 lead in the first set.

Williams held her serve to love and an easy missed volley into the net gifted the 17-times grand slam winner a break in the next, leading to Petkovic slamming her racket into thigh, floor and bag in turn.

In her first tournament match since the 2013 WTA Championships in October, Williams showed frustration throughout at the extent of her erratic play.

There were serves fired in at over 190 kilometres and 35 crunching winners, but even more mistakes.

After errant serves she would often practise her ball toss or technique, reinforcing the sessions spent with her father, Richard, in Florida during the off-season.

“I’m a renowned perfectionist - I think I wrote the book on being perfect,” Williams said. “I think it definitely helps me in a way.

“Also, it definitely holds me back at some moments. It’s just important for me to let the part that helps me kind of fill up and then the part that holds me back, just to let that go.”

Glaring Error

Her most glaring error came in the sixth game of the second set when a looping backhand from Petkovic landed just over the net to set up an easy winner but Williams was fooled by prodigious backspin and wildly missed the ball with a forehand. The crowd at Pat Rafter Arena roared with laughter and even Williams smiled, briefly.

Petkovic surrendered a 40-0 lead on serve in the fifth game of the second set, which the world number one ended up taking.

The German saved the first match point on her serve, but a forehand into the net in the following game gave Williams victory.

Williams next plays ninth-seed Slovakian Daniele Cibulkova who defeated Japanese veteran Kimiko Date-Krumm 6-3 1-6 6-3.

Williams’s slated semi-final opponent Maria Sharapova has a bye to the last eight after Australian Ashleigh Barty pulled out with an adductor injury.

On the other side of the draw, fourth seed Jelena Jankovic beat Ukrainian Elina Svitolina 6-1, 6-3, keeping the Serb on course for a semi-final meeting with defending Australian Open champion Victoria Azarenka.

In the men’s tournament, home favourite Lleyton Hewitt held off a spirited challenge from young compatriot and qualifier Thanasi Kokkinakis.

The 17-year-old saved 10 of 12 break points in the first set before succumbing 6-3 and was 5-3 up in the second, only for the veteran to win four straight games to take the match.

The twice grand slam winner faces sixth-seed Feliciano Lopez in the third round, after the Spaniard defeated Mikhail Kukushkin of Kazakhstan in straight sets.