(Adds Hewitt victory) By Ciaran Baynes BRISBANE, Dec 31 (Reuters) – World number one Serena Williams looked rusty in her first competitive match of the season ahead of the Australian Open but battled through to beat Andrea Petkovic 6-4 6-4 at the Brisbane International on Tuesday. 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.