Wildcard Sabine Lisicki produced the shock of the tournament after defeating French Open champion Li Na 3-6 6-4 8-6 in the women's singles.
The German world number 62, who has suffered a string of injuries in the last 12 months, could not handle Li's groundstrokes in the first set.
But she hit back in the second and saved two match points at 5-3 in the third set before claiming victory.
Sixth seed Francesca Schiavone and 18th seed Ana Ivanovic both advanced.
Italian Schiavone recorded a 7-5 6-3 victory over Barbora Zahlavova Strycova and will play Austria's Tamira Paszek in the third round, while former world number one Ivanovic enjoyed a comfortable 6-3 6-0 win over Greece's Eleni Daniilidou.
However, the biggest story of the day was Lisicki's stunning win over world number four Li.
The 21-year-old has been beset by numerous injuries as well as illnesses in the last 12 months, including a lengthy ankle injury and appendicitis.
And she continued her excellent form in Birimingham last week, where she beat the in-form Daniela Hantuchova to win the Edbgaston championship, at SW19 as Li suffered only her second Grand Slam defeat this year.
Australian Open finalist Li had been 4-2 up in the third set and twice served for the match, but Lisicki displayed impressive resilience each time.
"I really had to start from zero after being on crutches for seven weeks so it just means so much to me, winning the title in Birmingham and getting the wild card here," a delighted Lisicki told BBC Sport.
"I appreciate it so much, to be back in Wimbledon. It's just a place that I love so much.
"The crowd really helped me when I was two match points down, I didn't realise it could get so loud in there!"
Li said: "I'm not tired, after losing a tough match it's sad but today both players played well. All you can say is unlucky. Nothing went wrong, both players played unbelievable, nothing wrong. Just unlucky. I can't do nothing for these two match points."
Asked if she felt more pressure as the French Open champion, Li said: "I didn't feel different, so I didn't feel pressure, but only change now is every opponent plays better. If Sabine stays like this she's number one in the world."
Lisicki will play Japanese qualifier and world number 133 Misaki Doi, who beat 2008 Wimbledon semi-finalist Zheng Jie 6-3 6-1.
As well as Li, 14th seed Anastasia Pavlyuchenkova was another seed to suffer in the second round, losing to fellow Russian Nadia Petrova.
Former world number three Petrova registered a 6-3 6-3 victory on Court 17, taking advantage of Pavlyuchenkova's 24 unforced errors.
Two-time Grand Slam winner Svetlana Kuznetsova, the 12th seed, demolished Romania's Alexandra Dulgheru 6-0 6-2, while 16th seed Julia Goerges of Germany overcame a tight first-set tie-break to beat France's Mathilde Johansson 7-6 (12-10) 6-2.
Belgium's 19th seed Yanina Wickmayer needed three sets to defeat Georgia's Anna Tatishvili 3-6 6-4 6-2 but Slovakia's 24th seed Dominika Cibulkova enjoyed a no-nonsense victory with a 6-1 6-2 win over Slovenia's Polona Hercog.