Britain's Heather Watson forced Maria Sharapova to battle for over three hours to reach the third round of the Toray Pan Pacific Open.
The 20-year-old took the first set on a tie-break and caused problems for her illustrious opponent throughout as she sought to follow up her upset of Sabine Lisicki on Monday.
But the world number two eventually prevailed 6-7 (7/9) 6-3 6-4 in three hours and 10 minutes.
Watson was broken in the first game of the match but hit straight back, breaking to love in the second.
Sharapova took a 6-5 lead but failed to serve out the set, serving consecutive double-faults before Watson forced a tie-break with her third break point. The Guernsey player then came from 3-0 down to take it 9-7.
Sharapova broke early in the second set but again Watson responded and the Russian had to battle to hold for a 3-2 lead as her fragile serve gave her more problems, though she won the game with her sixth ace of the match.
Three successive breaks followed, Sharapova with two of them to take control before serving out the set 6-3.
For the third set in succession, Watson lost her first service game - but again she found the same response, levelling at 1-1.
Sharapova recorded the third break in succession - each sealed by a double-fault - but Watson made it seven in eight games to level once more.
Sharapova broke for the eighth time in the match to lead 4-3 and, serving at 5-4, an ace set up two match points and she closed the match out at the first attempt.