Williams, who won her 18th grand slam singles title in New York earlier this month, started sluggishly and found herself 5-0 and 40-15 down to Spanish qualifier Silvia Soler Espinosa.
However, the world number one saved four set points in a brilliant fightback before taking the match 7-5 6-2 to set up a second round clash with another qualifier, Bulgarian Tsvetana Pironkova.
It was the American's first outing since retiring in the first set of her opening match against Alize Cornet at the Wuhan Open last week because of illness.
Also returning after an absence, albeit a longer one, was 14 times grand slam singles champion Nadal, who had been sidelined for over three months with a wrist injury.
The world number two teamed up with compatriot Pablo Andujar in the men's doubles but they lost 5-7 6-4 10-4 to Czech Tomas Berdych and American John Isner in his first match since losing in the last 16 at Wimbledon.
The Spaniard is the second seed in the men's singles draw and will face Richard Gasquet of France in first round action on the Beijing blue courts later in the week.
The 28-year-old has picked a tough tournament to mark his return with eight of the world's top 15 taking part in the ATP 500 outdoor hardcourt event with five spots still up for grabs at the season-ending World Tour Finals in November.
CILIC CRUISES
One of those hoping to join Nadal in London is Croatian Cilic, playing his first ATP tournament since winning a maiden grand slam at Flushing Meadows in early September, who brushed aside local wildcard Bai Yan 6-3 6-4 on Monday.
The Chinese world number 464 pushed the fourth seed for long periods on an empty stadium court but wasted his four break point opportunities on the Cilic serve.
By contrast, Cilic was able to capitalise on his unheralded opponent's mistakes with Bai's forehand faltering as the match progressed, allowing the Croatian to secure a break in each set to advance to the second round.
"It's important for me to keep focusing on the improvement of my game and try to recreate the game I was playing at the U.S. Open," Cilic said.
Serbia's Viktor Troicki also advance after beating Russia's Mikhail Youzhny 6-3 6-4 with either the third seeded Berdych or Spain's Feliciano Lopez awaiting him in the next round.
Troicki's compatriot, former world number one Ana Ivanovic, had a far easier day by swatting aside Swiss teenager Belinda Bencic 6-2 6-1 to advance to the second round of the women's draw.
Ivanovic currently holds the eighth and final spot on the WTA's Race to Singapore standings with four places still up for grabs for next month's season-ending event.
(Writing by Patrick Johnston in Singapore; Editing by John O'Brien)
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