The second-seeded Chinese Li beat Croatian qualifier Mirjana Lucic-Baroni 6-1, 6-2 in 1 hour 14 minutes for a place in the quarterfinals after a first round bye, facing only some resistance late in the first and early in the second set.
Li was one of seven people chosen for the Time cover last week as part of their 100 Most Influential people issue. The immensely popular Li was the first Asian to win a grand slam singles title two years ago in Paris, and is a two-time Australian Open runner-up.
"Of course I was surprised," she reiterated after Thursday's match, adding that she is keeping her feet firmly on the ground as "a wife and a good player."
The Porsche Grand Prix is the first major buildup for the French Open which start in late May. The four top seeds headed by Russian Maria Sharapova, the reigning French Open and Stuttgart champion, had byes into the second round.
Sharapova was set to play Czech Lucie Safarova later on Thursday.
Results from the fourth day of the WTA Stuttgart tournament on Thursday (x denotes seeding):
Second round
Angelique Kerber (GER x3) bt Anastasia Pavlyuchenkova (RUS) 6-0, 6-4
Yaroslava Shvedova (KAZ) bt Carla Surez (ESP) 7-5, 6-4
Na Li (CHN x2) bt Mirjana Lucic (CRO) 6-1, 6-2
Ana Ivanovic (SRB) bt Nadia Petrova (RUS x8) 6-4, 6-3
Petra Kvitova (CZE x5) bt Julia Goerges (GER) 2-6, 7-6(7/4), 6-2