Defending mixed doubles champions Melanie Oudin and Jack Sock continued their improbable run together at the US Open, extending their undefeated streak to six matches as they fought off four match points in their 4-6, 6-3, 15-13 first round win over No. 5 seeds Katarina Srebotnik and Nenad Zimonjic.

Playing in front of a standing room only crowd on Court 17, Sock appeared drained from his singles win earlier in the day and dropped his serve in the opening game. Oudin also dropped her serve at 2-4 to give the No. 5 seeds a double break. They made a brief run of momentum in breaking Srebotnik when she served for the set, but Zimonjic made good on their second attempt and slammed down a overhead to give the No. 5 seeds a one-set lead.

The fighting effort of the American pair was what helped them grab the mixed crown here last year, and they showed plenty of that same heart in the second set. One break of serve was all they needed when the Srebotnik double faulted to give the wild cards a 3-1 lead. The US team rode that slight lead all the way to the end of the set.

Both teams saved their best tennis for the super-tiebreaker at the end of the match that produced plenty of dazzling rallies. Oudin and Sock fought off a match point on Zimonjic's serve at 9-8 before wasting a match point of their own on Oudin's serve at 10-9. The American pair fought off a trio of match points down 10-11, 11-12 and 12-13, before a volley sent long from Srebotnik gave them a second match point. A forehand dumped into the net from Srebotnik brought the crowd to their feet as the Americans embraced in celebration.

In other doubles matches around the grounds, three-time US Open champ Kim Clijsters came one step closer to her retirement after losing her first round women's doubles match on Court 4. Playing with fellow Belgian Kirsten Flipkens, the pair bowed out meekly in a 6-3, 6-4 loss to Shuai Zhang of China and Chia-Jung Chuang of Chinese Taipei. Clijsters will now focus solely on mixed doubles when she plays her first round match with American Bob Bryan.

No. 1 seeds and defending champions Lisa Raymond and Liezel Huber managed to avoid a massive first round upset on that court earlier in the day, fighting back from 0-3 down in the final set to defeat Aussie Casey Dellacqua and Eleni Danilidou of Greece 6-4, 6-7, 6-4. The American pair are coming off a tournament win last week in New Haven, their first title together in nearly six months.

Follow usopen.org staff writer McCarton Ackerman on twitter at @mccartontweets

• Nenad Zimonjic Bio
• Katarina Srebotnik Bio
• Jack Sock Bio
• Lisa Raymond Bio
• Melanie Oudin Bio
• Liezel Huber Bio
• Kim Clijsters Bio