Halep took time out to head to the stands and watch the Williams sisters face off for the 29th time in their career in the ongoing Indian Wells WTA tournament.
Venus Williams defeated sister Serena 6-3, 6-4 in the third round for her first win over her younger sibling since 2014.
Serena leads Venus 17-11 all-time and has won eight of their last nine meetings, most recently the Australian Open final last year.
Williams' only competitive appearances since the birth came in December at an exhibition in Abu Dhabi, a Fed Cup doubles match with sister Venus last month and an exhibition in New York on Monday.
Serena, a winner of 23 Grand Slam titles, is in the same quarter of the draw as older sister Venus, the siblings could potentially face each other in the third round of the tournament.
The Williams sisters were the headliners Monday night in the Tie Break Tens tournament in Madison Square Garden, an eight-woman event in which all matches were 10-point tiebreakers, instead of the usual games and sets.
In Serena's first competition since giving birth five months ago, she and sister Venus lost 6-2, 6-3 to Lesley Kerkhove and Demi Schuurs of the Netherlands in a Fed Cup doubles match.
Williams teams up with Lauren Davis to face Lesley Kerkhove and Demi Schuurs of the Netherlands in Sunday's doubles match.
The 19-time major winner can afford to relax slightly longer, given the half of the draw that he shares with Djokovic doesn't start until day two of the Australian Open.
Former world number one Novak Djokovic, who is making a comeback after a six-month layoff due to a tennis elbow injury, is ranked 14 in the first major of the season.
Azarenka hasn't competed anywhere since Wimbledon, where she lost to Simona Halep in the fourth round on July 10.
Serena's withdrawal comes less than 24 hours after fellow former world No.1 Andy Murray withdrew from the men's event with a chronic hip injury.
Williams said after the 67-minute match at the Mubadala World Tennis Championship that she is still undecided on her title defence of the Australian Open, which starts Jan. 15 but delivered a warning to rivals.
Williams will face Latvia's Ostapenko at the Mubadala World Tennis Championship in the first tournament of the year that women will take part in, the organizers announced on Sunday.
The former world number has not played any competition for a long time and is likely to make a comeback in Melbourne for the first time since giving birth to a baby girl in September.
In September, Serena had revealed that she had set an 'outrageous' goal for making a comeback at the Australian Open.
Tournament director Craig Tiley is optimistic that she would return for a crack at her seventh Melbourne Park crown.
The US tennis star and her new hubby give the world a glance of their 'secret' wedding on social media.
Many of the 23-time Grand Slam winner's BFF attended the wedding extravaganza.
The American tennis star and Ohanian, who is the co-founder and executive chairman of Reddit, plan to tie the knot in an intimate ceremony at New Orleans' Contemporary Arts Center.
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