Watson won the 2009 US Open Junior title
British teenager Heather Watson moved through to round two of the Eastbourne International with a 6-3 6-2 win over world number 48 Aleksandra Wozniak.
It was a first WTA main draw victory for the 18-year-old, who is a wildcard entry for Wimbledon next week.
Defending champion Caroline Wozniacki and French Open winner Francesca Schiavone both suffered shock defeats.
British number one Elena Baltacha lost a first set tie-break to Li Na but went through after the seventh seed retired.
Baltacha was beaten by Li - who won the Aegon Classic in Birmingham on Sunday - in the first round last year but went 2-0 up before the Chinese player fought back to win the tie-break 8-6.
However, Li failed to appear for the second set after treatment on a leg injury, giving Baltacha her second victory over a world top-10 player.
Baltacha, who was forced to retire from her first-round clash in Birmingham with heatstroke last week, will now face Li's compatriot Zheng Jie, who beat Shahar Peer 1-6 6-1 6-3.
I've just been playing so well and it has made me really happy, and it shows me that I can compete with these girls
Guernsey's Watson had already beaten world number 81 Tsvetana Pironkova in one of three qualifying wins to reach the main draw, and will now play world number 15 Victoria Azarenka.
"I'm so, so happy," said Watson, the world number 344. "I was really happy to get through qualifying but I didn't want to stop there. She's a very good player, she came through three qualifying matches, so I knew I'd have to work hard.
"I love playing in front of a crowd, it's so much fun, especially at home and I just want to get through a few more rounds and the ranking points will also be very helpful.
"I've got a big player next round. She's very experienced and I'll give it my all. I've just been playing so well and it has made me really happy, and it shows me that I can compete with these girls."
Azarenka, of Belarus, caused an upset of her own by beating fourth seed Agnieszka Radwanska of Poland 7-6 (7-2) 6-1.
Kim Clijsters outplayed her 20-year-old fellow Belgian Yanina Wickmayer, completing a 6-1 6-1 victory in 54 minutes in her first grass-court tournament for almost four years.
The 27-year-old US Open champion last played in a grass event at the 2006 Wimbledon semi-finals where she was beaten by compatriot Justine Henin.
World number three Wozniacki struggled with the blustery conditions and the attacking, powerful game of France's Aravane Rezai, who won 6-4 1-6 6-3.
"She played well, she tried to take the ball early and hit every shot," Wozniacki said.
"Definitely I would like to have got some more matches here but I just need to keep my head up and play some practise matches and prepare for Wimbledon that way," added the 19-year-old, who reached the fourth round at SW19 last year.
Second seed Schiavone was beaten 7-5 6-3 by Sorana Cirstea of Romania in her first match since defeating Sam Stosur to claim the French title at Roland Garros.