Lindsay Davenport saved two match points to beat Daniela Hantuchova of Slovakia 3-6, 7-5, 6-2 and reach the quarter-finals of the Zurich Open on Thursday.
The American's victory means she will begin an 85th week as the world number one on Monday.
It will be her eighth period as the top-ranked player after first claiming the spot in October 1998.
Earlier, France's Amelie Mauresmo suffered her second embarrassing defeat in two weeks after she was humbled 6-2, 6-0 by Slovenian qualifier Katarina Srebotnik.
Last week in Moscow, Mauresmo was beaten 6-1, 6-1 in her opening match by Francesca Schiavone of Italy.
Davenport, the top seed, looked ragged for most of the match with Hantuchova, committing a succession of errors.
"I knew it would be tough," said Davenport. "I didn't feel I was playing my best or striking the ball that well, but I still managed to win despite the circumstances."
After claiming the first set and earning an early break in the second, Hantuchova held two match points at 5-4.
But Davenport struck a backhand volley winner on the first and Hantuchova hit a forehand long on the second to bring the game to deuce. Davenport then broke for 5-5 with a crosscourt return and a double-fault.
EASY FOREHAND
Davenport had to hold off two break points in the next game, but Hantuchova put an easy forehand wide and then made another forehand error to give Davenport a further break and the set.
Davenport began the final set by breaking her deflated opponent to love to lead 2-0 and although Hantuchova broke back immediately she handed Davenport the lead once more by double-faulting when down 3-2.
Davenport then gave up just two points in the final two games.
The American said: "She became a little bit emotional when she didn't win those match points and seemed really down even starting at 5-5, so I was just trying to take advantage and move forward."
Mauresmo, tired at the end of a long season, was overwhelmed by an opponent who had not claimed a set from her in their previous three meetings. Looking flat, she allowed 43rd-ranked Srebotnik to dictate play.
The Frenchwoman said: "There's no fun for me, for the crowd. There's no point for me considering what happened last week and today to keep going and pushing for no result."
Srebotnik, a former Wimbledon junior champion, said: "She can probably say now she was tired, but all the players are tired. I have four matches behind me, plus two doubles, and physically I'm also tired.
"I think today that I really played better than she did, because everything she tried I had an answer for."
