Best friends shared the same side of the net, while the ATP Cup saw its second team qualify for the knockout stage. These are the Top 5 photos from Monday's action. (All photos by Getty Images)

1

Serena Williams and Caroline Wozniacki kicked off their 2020 season with a special opening-round win in Auckland. The close friends teamed up for the first time and posted a 6-2, 6-4 victory against Nao Hibino and Makoto Ninomiya. "I can’t let the rest of both of our amazing careers go without playing with her," said Serena. Wozniacki is set to retire following the Australian Open.

Top 5 Photos, 1/6: 
Serena & Wozniacki
win team debut

Top 5 Photos, 1/6: Serena & Wozniacki win team debut

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2

Dominic Thiem boosted Team Austria to a 3-0 triumph over the Argentines. The world No. 4 dismissed 2019 US Open quarterfinalist Diego Schwartzman in straight sets to seal the victory. After starting his season with a loss to Borna Coric, Thiem stated, "I'm really happy with the improvement from Saturday to today."

Top 5 Photos, 1/6: 
Serena & Wozniacki
win team debut

Top 5 Photos, 1/6: Serena & Wozniacki win team debut

3

Team Serbia became the second squad to secure their spot in the final eight after inching past France. Novak Djokovic ousted Gael Monfils, 6-3, 6-2, to move to 16-0 in their head-to-head series. Djokovic then joined forces with Viktor Troicki to topple last year's Wimbledon finalists Nicolas Mahut and Edouard Roger-Vasselin, 6-3, 6-7 (5), [10-3], to clinch the 2-1 victory.

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Top 5 Photos, 1/6: 
Serena & Wozniacki
win team debut

Top 5 Photos, 1/6: Serena & Wozniacki win team debut

4

In just over an hour, Coco Gauff dispatched Viktoria Kuzmova, 6-3, 6-1, to advance in Auckland. The Linz champion rolled past the world No. 61 Kuzmova by using her consistency and precise ball movement. The 15-year-old will either face Alison Van Uytvanck or Laura Siegemund for a spot in the quarterfinals, where she could potentially meet Serena Williams.

Top 5 Photos, 1/6: 
Serena & Wozniacki
win team debut

Top 5 Photos, 1/6: Serena & Wozniacki win team debut

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5

After practicing together over the weekend in Doha, Stan Wawrinka and Frances Tiafoe maintained their cheerful mood in their 6-3 7-6 (6) win against Alexander Bublik and Andrey Rublev. The tandem will next challenge No. 3 seeds Wesley Koolhof and Rohan Bopanna​ for a spot in the semifinals. ​

Top 5 Photos, 1/6: 
Serena & Wozniacki
win team debut

Top 5 Photos, 1/6: Serena & Wozniacki win team debut