Serena holds off challenge

Competing in only her third event of the year because of a knee injury, Williams dominated the last two sets in sapping heat after losing the first 7-4 on a tiebreak.

The former world number one, who had to rally from a set down to beat Slovakia's Daniela Hantuchova in the previous round, will meet Ana Ivanovic or fellow Serbian Jelena Jankovic in the last four.

Unseeded Shaughnessy, beaten in straight sets when the pair last met in the 2003 Australian Open quarter-finals, survived four break points in the opening set before coming back from 4-2 down in the tiebreak.

The 27-year-old from Scottsdale in Arizona held serve to level at 4-4 and then powered a two-fisted backhand down the line to edge ahead 5-4.

After her opponent hit a forehand long, Shaughnessy unleashed a thunderous first serve to force another Williams error and clinch the first set.

However, the twice champion then upped the tempo.

Generating impressive power with her groundstrokes and advancing to the net whenever possible, Williams broke Shaughnessy three times in the second set to level the match.

The deciding set went with serve until the eighth game when Williams again broke after working Shaughnessy around the court with superbly crafted rallies.

Serving for the match at 5-3, Williams was surprisingly broken for the first time after delivering her first double-fault.

Stung by her lapse, Williams took advantage of weak second serves by Shaughnessy and immediately broke back to end the encounter, her opponent double-faulting on match point.

In the second quarter-final, 10th-seeded Ivanovic was due to meet compatriot Jankovic, followed by third-seeded Russian Elena Dementieva against American wildcard Bethanie Mattek.

Top seed Maria Sharapova, chasing her second consecutive WTA Tour title, was scheduled to take on fellow Russian Dinara Safina in the day's late match.