Number two seed Ostapenko of Latvia lost to the big-serving Czech Krystina Pliskova 6-1, 6-4 in just an hour and 20 minutes.
Ostapenko had beaten Williams via a super tie-break 6-2, 3-6, 10-5 in Abu Dhabi on Saturday, but she had no answer to the power of Pliskova, identical twin sister of former world number one Karolina.
“I felt a bit nervous in the second set, but my serve was really working today, which was a big help,” Pliskova said after the match.
Pliskova, third on the WTA Tour’s ace charts last season, raced out to a 5-0 lead to start the match.
“My serve was great today, especially in the first set. I feel like it’s really fast here, so that can be tricky for both of us, and the first of us playing fast won the point.”
It was not the start to 2018 that Ostapenko wanted, with the first Grand Slam of the year, the Australian Open, starting in 13 days’ time.
Earlier there had been disappointment for the home crowd when Chinese number three seed Zhang Shuai was bundled out in three sets by Kazakhstan’s Zarina Diyas.