MELBOURNE, Australia -- Aussie heroine Samantha Stosur and Zhang Shuai of China earned a tight 7-6(2), 7-6(4) victory over No.1-seeded Czechs Barbora Krejcikova and Katerina Siniakova, to seal their spot at the top of the 2019 Australian Open semifinal draw.

The stunning result is representative of how the latter stages of the women's doubles event concluded, as three unseeded teams join the second-seeded defending champions in the final two rounds in Melbourne. Despite their lack of seeding, however, Stosur and Zhang have reached the semifinals in two straight Grand Slam events, matching their result at the 2018 US Open.

Zhang and Stosur, close friends off the court as well as doubles partners this week, gritted out the upset over the reigning French Open and Wimbledon champions in just over two hours. The unseeded duo came back from a break down in each set, using their powerful groundstrokes to edge the net-rushing Czechs.

Three consecutive breaks opened the first set before the top seeds started to excel at the net, easing to a single-break lead at 3-1. The Czechs then held that advantage up to 5-4, but with Krejcikova serving for the set, Zhang began to fire on all cylinders, crushing a crosscourt return to set up break point, then breaking with a winning volley.

After a hold at love by the Chinese player, Siniakova needed to survive a marathon game on her serve at 6-5, staving off four break points before holding with an ace. But in the tiebreak, Krejcikova plopped an overhead into the net to give Stosur and Zhang a 4-2 lead, and the Australian/Chinese pairing rolled through the rest of the breaker from there, with Stosur finishing the set with a forehand winner.

The top seeds also had a chance to serve out the second set on Siniakova's delivery at 5-3, but Zhang again gave her team a break point with a return winner, and Stosur followed that with a forehand return winner of her own to get back on serve.

After a tough hold by Stosur for 5-5, Krejcikova was broken, giving Zhang a chance to serve out the match, but she was broken as well without reaching match point.

In the second-set tiebreak, the duos were deadlocked up to 4-4 before Stosur crushed a forehand return winner down the line off of the Siniakova serve, giving her team a 5-4 lead with two service points for the Australian to come. Stosur and Zhang won those two points to notch the upset in front of a gleeful crowd on Margaret Court Arena.

Defending champions and No.2 seeds Timea Babos of Hungary and Kristina Mladenovic of France were more fortunate in their quarterfinal match, as they overcame No.9 seeds Raquel Atawo of the United States and Katarina Srebotnik of Slovenia, 6-4, 7-5 in 75 minutes.

The Hungarian/French tandem have been excellent at the hardcourt Slams since they reformed their pairing at the start of 2018. The reigning titleholders in Melbourne also reached the 2018 US Open final before falling to Ashleigh Barty and CoCo Vandeweghe.

Babos and Mladenovic will face unseeded Jennifer Brady and Alison Riske in the semifinals, as the all-American duo picked up their second upset in a row with a 6-3, 7-6(3) victory over the 7th-seeded Chan sisters, Latisha and Hao-ching, of Chinese Taipei.

Brady and Riske, who upset No.4 seeds Nicole Melichar and Kveta Peschke in the previous round, served for the match at 6-3, 6-5, before the Chan sisters broke via a winning volley by Latisha. But strong serving and well-placed returns by the Americans brought them to a commanding 6-1 lead in the tiebreak, and they held on from there for another upset win.

Finally, despite the top seeds going down, all was not lost for the Czechs, as unseeded Barbora Strycova and Marketa Vondrousova claimed the last spot in the semifinals with a 6-4, 6-4 win over No.5 seeds Andreja Klepac of Slovenia and María José Martínez Sánchez of Spain. Strycova and Vondrousova will take on Stosur and Zhang in an all-unseeded semifinal.