The French Open champion’s performance bordered on the farcical at times, carelessly frittering away three set points to surrender the initiative before pulling off an escape act.
Second seed Sharapova left herself with work to do after a string of double-faults and horror misses gift-wrapped the first set tiebreak to Watson 9-7.
The errors continued to mount but the Russian, her squealing growing increasingly louder with the effort, did just enough to restore parity by winning the second set.
“I definitely felt rusty today,” said Sharapova, who served 14 double-faults and finished with 67 unforced errors. “It’s one of those matches you’re just happy to get through.”
One of three Chinese players who made the trip to Tokyo despite a blazing political row between her country and Japan, Li’s experience told after a wobble in the second set.
“I’m still not 100 percent,” Li told Reuters. “Just trying to stay healthy and taking (cough) medicine every morning and every night.”
Li’s appearance in Tokyo was a relief for organizers after China withdrew its badminton players from last week’s Japan Open amid tensions over disputed islands in the East China Sea.
Playing on an outside court, Li lacked sharpness after a whirlwind book-signing tour in China following the release of her autobiography.
However, the 30-year-old saved her best for the big points, putting the match away by smashing a cross-court backhand into the corner after two hours, 37 minutes.
Former Wimbledon champion Petra Kvitova was bundled out in the first upset of the WTA premier five event, suffering a 6-4 6-4 defeat by Croatia’s Petra Martic.
The fourth seed squandered numerous opportunities against her 72nd-ranked opponent, who closed out the match with a fierce serve which almost knocked the Czech over.
“That was the biggest win of my career so far,” smiled Martic. “The last five or six days have been really weird. I’ve played bad in practice and just couldn’t do anything.”
Martic next faces Russia’s Nadia Petrova after the 17th seed beat Romania’s Simona Halep 7-6 6-0.
Nine of the world’s top 10 women are appearing in the hardcourt tournament, U.S. Open champion Serena Williams the only player absent.
Germany’s Angelique Kerber made short work of Japanese wildcard Ayumi Morita, the fifth seed comfortably winning 6-3 6-4 to reach the third round.
Sixth seed Sara Errani of Italy beat Sweden’s Johanna Larsson 6-3 7-6 and next faces Frenchwoman Marion Bartoli after the ninth seed’s 6-2 7-5 win over German Julia Goerges.
(Editing by John O’Brien)