ABU DHABI, UAE - Less than four months after becoming a mother, defending Australian Open champion Serena Williams kicked off her 2018 against the highest caliber of opposition on Saturday: a fellow Grand Slam winner.

While not an officially sanctioned WTA match, two of last season's major champions squared off on the final day of the 10th annual Mubadala World Tennis Championships, traditionally an exhibition event for male players in Abu Dhabi.

This year's edition featured two WTA stars for the first time, as the American took on World No.7 and defending French Open champion Jelena Ostapenko.

Ostapenko won Saturday's exhibition in a deciding match tiebreak, 6-2, 3-6, 10-5.

Takeaway: Three thoughts on Serena Williams' return in Abu Dhabi

"I was a little worried out there. I looked at my camp and was like, 'Is Olympia okay?', Williams said, referencing infant daughter Alexis Olympia Ohanian, Jr., on-court after the match.

"It's really good to be back on court. It's my first time playing in Abu Dhabi...first matches back are always super incredibly hard, but it was great and I'm glad I could do it here."

Jelena Ostapenko in action in Abu Dhabi. (Mubadala World Tennis Championships)

The 20-year-old Latvian told reporters earlier in the week that it was an "honor" to play the 23-time Grand Slam champion in her first match as the latter embarks on a comeback.

"I think it’s such a great opportunity," Ostapenko told Sport360's Reem Abulleil on Friday. "She was my idol when I was growing up and I really liked the style she was playing and I think she’s very big for women’s tennis."

Williams, currently ranked World No.22, is entered to defend her Australian Open title in just over two weeks. 

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The former World No.1 and sister Venus hosted a private on-court charity event earlier in the December, but Saturday's match marked the first time that Williams was on court in a public, competitive setting.