Williams admitted her mind wandered to her child, Alexis Olympia Ohanian Jr., during the match.
Against Ostapenko on Saturday, Williams struggled to contain her serve as her Latvian opponent won 6-2, 3-6 and then 10-5 in a super tiebreaker.
World No 7 Ostapenko, who enjoyed a superb 2017 season that included winning her first grand slam title at the French Open at Roland Garros, said it was fantastic to play Williams.
"I'm excited. It's good to be back on the court, this is such a good time for me", Williams, 36, told Eurosport.
And Bautista Agut is looking forward to welcoming him back to match-play when the pair collide. This will be the first time that women have taken part in the event and it is a huge honour to be part of that history after watching the tournament for many years. "I am going to assess everything with my team before deciding".
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Federer won the Australian Open and a record eighth Wimbledon while Nadal claimed a 10th French Open and the US Open in a Slam sweep for the two veteran stars. "I want to become a more consistent player in 2018", she added. But she was nowhere near her best in the first set before fighting back and winning the second.
World number 20 Bautista Agut, a two-time victor on the ATP Tour in 2017, was broken when serving for the first set at 5-3 and then saw three set points come and go in the next game. "The more I played, the more confident I felt that I would be able to go for shots that I was afraid to go for in the first set".
This will be the first-time women players have played in the Championship.
"It is dubbed an "exhibition", but the tennis this week in Abu Dhabi is great - it is very competitive and everyone is going to give it their all".
The men's field in Abu Dhabi features US Open semi-finalist Pablo Carreno Busta, Spain's Roberto Bautista Agut and 20-year-old Russian Andrey Rublev apart from Djokovic, Thiem and Anderson.