Elena Baltacha fought back from a set down to beat Alona Bondarenko 3-6 6-3 6-4 and become the only British woman to reach the second round at Wimbledon.
Baltacha, 25, ranked 73 places below the Ukrainian, began erratically and Bondarenko easily took the first set.
But the world number 33 soon lost her way and Baltacha took full advantage to clinch a memorable victory.
The British number two plays Kirsten Flipkens in round two after the Belgian beat 30th seed Agnes Szavay 7-5 6-4.
Baltacha, who reached the third round here in 2002, was forced to wait until after 7pm before starting her match on a half-empty Court One.
Baltacha hails 'best win of career'
Initially she looked out-of-sorts, with her big forehand particularly unreliable and frequently flying long.
But Baltacha settled as the match wore on, while Bondarenko began to look increasingly nervous.
In the second set, it was the former world number 19 who was making all the errors, while the Scot's groundstrokes were now finding their range.
The third set was a tense affair, with Baltacha twice going a break up to the delight of a vociferous crowd before being pegged back to 2-2 and 4-4.
But Baltacha found some of her best serves of the match to move 5-4 clear before breaking again to complete her biggest win since she overcame 32nd seed Amanda Coetzer in the second round at Wimbledon seven years ago.
"I'd say Coetzer was ranked 32 or the 32nd seed but at that time she was on her way out her age and pretty much retired that year or the next," she said. "Alona has been on good form and she made the finals of Warsaw so this is a very big win for me."
The victory also propels Baltacha into the top 100 in the rankings and she said: "I knew I would get there if I won going into the match so it was a very important match for me.
"I will also get into the main draw in the US Open and I am not playing anything after this week.
"I knew at the same time where I am with my tennis so there was nothing for me to lose and everything for me to gain.
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