But Williams was determined to get home in time to put her daughter to bed, reeling off five successive games to get the job done in straight sets, eventually winning on her sixth match point.
Opening the 2018 Wimbledon Championships on Centre Court, men's top seed and defending champion Roger Federer wasted little energy in dispatching Serbian Dusan Lajovic 6-1 6-3 6-4 in 79 minutes.
"If it's the shorter days, now that I'm not playing doubles in this event, I have the day off, I think that will help".
Back in 2011, there was the freaky situation of Williams hiding in her panic room when testers arrived at her LA mansion, dialling 911 as she believed them to be intruders.
Last year's runner-up Marin Cilic also advanced easily with a 6-1, 6-4, 6-4 win over Japanese player Yoshihito Nishioka.
She is competing in her second grand slam since giving birth, having reached the fourth round of the French Open before withdrawing with a pectoral injury ahead of a match against old rival Maria Sharapova.
"I'm very happy, I felt good from the start too which was nice and that was not the case previous year against him".
The American returns to the tournament for the first time since claiming a seventh title in 2016 having missed previous year to give birth to her daughter.
She added: "Tennis has given me so much. I really don't like being away from her", Williams said. "I remember I struggled early on a lot", said Federer who hardly broke sweat in Monday's 30-degree hothouse. So I'm just not used to that yet. Just due to the numbers, it looks like I'm being pushed out.
Williams, looking to move within one title of Martina Navratilova's Open era record of nine Wimbledon titles, fired 23 winners past Rus.
The 25th seed had to battle from a break down in the second set but went on to win five games in a row to set up a second-round encounter against either Tereza Smitkova or Viktoriya Tomova. "I came in here expecting that maybe I wouldn't get a seed".
In the women's draw, Serena's sister Venus overcame a slow start to claim a 6-7 (3) 6-2 6-1 win over Swede Johanna Larsson. I'm happy to get through because I didn't play my best.
Australian Open champion Caroline Wozniacki needed less than an hour to book her second-round spot, beating American Varvara Lepchenko 6-0 6-3. It also is not unusual for athletes who have taken long breaks to be subject to additional testing because of natural physiological changes that might have occurred during the time off. "I've been tested quite a bit, quite frequently out of competition".
"I do know I'm always tested, I'm always getting tested, all the time".
However, 2017 semi-finalist Magdalena Rybarikova lost 7-5, 6-3 to Sorana Cirstea.
A few weeks earlier, Williams had taken to Twitter to vent her frustrations: "And.... just like that anti doping is here ...again... second time this week".
Serena's fellow tennis-playing mother, Victoria Azarenka, a former world number one and two-time semi-finalist, defeated Ekaterina Alexandrova of Russian Federation 7-6 (7/4), 6-3.
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