British number three Anne Keothavong put on a superb display to beat former world number one Ana Ivanovic 6-3 6-2 in the Luxembourg Open second round.
The 28-year-old world number 93 took only 74 minutes to oust the Serbian, who is now ranked 23.
Keothavong, whose ex-coach Nigel Sears now works with Ivanovic, said on Twitter: "It feels great to beat a former world number one. I'm chuffed."
The Londoner now goes on to face Dutch qualifier Bibiane Schoofs.
And she admitted it was strange to have Sears, who quit as the LTA's head of women's tennis in June, helping out an opponent.
"It felt a little weird too seeing someone who has helped me so much with my tennis over the years sitting in the opposition's box," she added.
Elsewhere, top seed Victoria Azarenka of Belarus beat Italy's Alberta Brianti 6-0 6-1.
Keothavong will play Schoofs in Friday's second quarter-final at around 1330 BST.