Dellacqua went down 7-5, 4-6, 6-3 to the 55th-ranked Riske in windy conditions at the Domain Tennis Centre. 

Riske broke the West Australian three times to Dellacqua's two in the final set, overcoming two double faults and a break point in the final game as she served out the match. 

The 23-year-old American, who made the fourth round at the US Open last year, will play China's Zhang Shuai or Czech Klara Zakopalova in the quarter-finals. 

Dellacqua, who made three grand slam finals with doubles partner Ashleigh Barty last year, beat Belgian qualifier Alison Van Uytvanck in the first round. 

The 28-year-old left-hander will now head to Melbourne looking for a first major doubles title with the young Australian. 

In a first set that included six consecutive breaks of serve, it was Riske who got the crucial one when leading 6-5. 

The American converted her first set point with a sizzling forehand down the line to take the set 7-5. 

Dellacqua broke Riske twice in the second and was down 0-40 serving for the set at 5-4. 

She won four straight points to take it 6-4 when Riske netted a first set point. 

In first round matches, Australian teenager Storm Sanders marked her WTA main draw debut with a 6-2, 6-2 thrashing of world No.39 Peng Shuai. 

The 19-year-old is ranked No.234 in the world, 195 places behind world No.39 Peng. 

Sanders will play Wimbledon semi-finalist Kirsten Flipkens, the second seed, in the next round. 

Defending champion Elena Vesnina progressed to the second round with a 6-4, 6-4 win over Puerto Rican Monica Puig. 

Australian wildcard Olivia Rogowska has crashed out at the hands of eighth seed Bojana Jovanovski. 

Rogowska looked in control when she sent down an ace to take the first set but fell away badly to go down 3-6, 6-2, 6-0 in the second round.