SHENZHEN, China - Reigning Grand Slam champions went 1-1 on Monday in doubles round robin action in the Red Group at the Shiseido WTA Finals Shenzhen. 

French Open champions and No.3 seeds Timea Babos and Kristina Mladenovic kicked off the session of play with a 6-2, 5-7, 10-7 victory over No. 5 seeds Latisha Chan and Chan Hao-ching, before No.8 seeds Anna-Lena Groenefeld and Demi Schuurs sprung a surprise on US Open champions and top seeds, Elise Mertens and Aryna Sabalenka, 7-5, 1-6, 10-7.

The defending champions at the WTA Finals from the final edition in Singapore, Babos and Mladenovic were up a break three times in the second set, and held two match points in the 10th game, but ultimately came back from an early 3-1 deficit in the match tiebreak.

"It was a very, very difficult start," Babos said after the match. "Every girl says that it's not easy to start a tournament with this format, because you have to play your best from the very beginning.

"I think we did a really good job from the first point to the last. The sisters played some great tennis towards the end of the match, and I'm just very, very pleased, happy and even proud with how we handled this situation.

"We could've been disappointed with not winning in two sets, but I said to Kiki, 'One hour later, we got it done,' so that's the most important."

The Chan sisters, playing as a pair at the season-ending event for the first time, later leveled the tiebreak at 6-6 from 6-4 down, but lost the final four points to allow Babos and Mladenovic to come away with a win.

"They really turned it around at some point, and picked up their level," Mladenovic added. "They...kept their chances to come back.

"I think overall, it was a very solid and great match from us. You can be tight, or not really going for your shots and doing the right things and hitting big, and we did that really well. I felt like we were loose and pumped up to play our best game.

"It was just about hanging in there and staying strong, and I think we did that well in the third set."

Defending WTA Finals doubles champions Timea Babos and Kristina Mladenovic kicked off the Red Group with a win. (Getty)

The German and Dutch duo capped off Monday's play with a big victory against the overall top seeds, with in-form Sabalenka having lifted the singles trophy last week at the WTA Elite Trophy in Zhuhai.

The No.8 seeds reversed the result of the pair's semifinal meeting from the Dongfeng Motor Wuhan Open earlier in the Asian swing, winning three straight games to close the first set after they twice lost a break lead. 

In the deciding tiebreak, Groenefeld and Schuurs sprinted out of the gates, and won six of the first seven points, but had to hold their nerve down the stretch as the US Open champions got the tiebreak back on serve from a 7-3 deficit. 

"We knew it was going to be a tough match. We played them a few weeks ago in Wuhan, and they beat us easily with the score, but we knew it was a close match because we lost a lot of no-ad points," Groenefeld said.

"We went out today and tried to be aggressive, and we're very happy with the performance tonight."

Groenefeld and Schuurs scored an upset on Monday in Shenzhen. (Getty)

The opening victory proved doubly sweet for Schuurs: having qualfied for the WTA Finals a year ago alongside Mertens, bowing out in the quarterfinals, both the 26-year-old Dutchwoman and Groenefeld scored their first-ever wins at the event.

"I know Elise very well," Schuurs said with a laugh, "but she knows me as well. It's always a close match. In Wuhan, we played 6-1, 6-2, but today, we beat them. 

"We played a great match. In the second set, we lost it a little bit, and they started to play a bit better. Elise is still a good friend of mine, but today was our day, and that's good."

Having competed in doubles at the WTA Finals under the previous knockout format, the No.8 seeds can now look ahead as they look to keep their Shenzhen campaign rolling. 

"It's the first time for both of us that we get to play in this format," the German said. "It's very exciting and to start it off with a win is always nice, so now we go with a lot of confidence into the next match."