The American, 31, started poorly, then won four games in a row before allowing herself to be broken twice while serving for the first set in a sluggish, error-strewn performance that saw her serve terribly in the first set.
China’s Li, 30, fared significantly worse though, making some truly horrendous gaffes as she failed to capitalise on the world number three’s malaise to slip to a 1-6 record against this year's Wimbledon, Olympic and US Open champion.
Breaking was the rule and not the exception in the first set, with a frustrated Williams twice failing to serve it out, more by her own design than any fine play by Li.
The American was struggling to find accuracy with her trademark power deliveries, but Li was blundering away her own service games with double faults and unconscionable mistakes, blasting a series of winning volleys and drives long and wide when her opponent – whose movement seemed restricted – was stranded.
Serena pulled herself together in the first set tie-break, which she won fairly comfortably, before finding her serve early in the second set as she unleashed a series of aces, before the Chinese double-faulted and mis-hit a smash to lose the first break of the set.
Li did break back as Serena showed that she too could double fault under pressure, but the rest of the set was the American’s as she came closer to the form that saw her toss Angelique Kerber aside in the opener on Tuesday.
Ultimately Williams was in charge as soon as she re-broke to 3-1, again the result of a dreadful double-fault from Li, whose second serve down the middle almost hit the baseline it was so long, defending serve with ease and almost breaking to win at 5-2 as the 2011 French Open champion saved two match points.
There was no joy for the Chinese when she had to break to stay in it as Williams blasted her way to a love hold and a second win in two matches.
Williams is all-but through now, while this was Li’s first match after squeaking into the finals with a late-season upsurge in form.
Next in the Red Group Victoria Azarenka faces Kerber, the world number one’s first match of the Championships.
- Sports & Recreation