Serena Williams put a "crazy" few games behind her to beat Julia Vakulenko of Ukraine 7-5, 6-2 and reach the quarter-finals of the Stuttgart Grand Prix on Wednesday.
Williams, the Australian Open champion and sixth seed, was leading 4-1 in the first set when she let her concentration slip, allowing her opponent to get back level.
The American recovered to finish off the first set in 40 minutes and proved too strong in the second, settling the match by breaking to love in the final game.
"I went a little bit crazy out there but it was important to keep positive," Williams said on court after the match.
She goes on to face Svetlana Kuznetsova in the last eight of the $650,000 indoor tournament after the Russian second seed won a scrap with Poland's Agnieszka Radwanska 6-4, 6-7, 6-3.
Kuznetsova served superbly throughout but after edging the first set she missed key breakpoint chances in the second.
Radwanska survived to a tiebreak, which she won 7-5, before Kuznetsova reasserted herself to move 5-0 ahead in the decider.
The world number two missed a chance to serve out at 5-1 but she made it at the second attempt.
"She was playing so well, really above herself in the second set and I knew she couldn't keep that up," Kuznetsova said at a news conference. "If she had done so, bravo to her."
There was an upset in the other second-round match on centre-court at the Porsche Arena when Tatiana Golovin of France beat fourth seeded Russian Anna Chakvetadze 7-6, 6-1.
Golovin won the tiebreak 7-3, after her opponent had double-faulted on setpoint, and ran away with the second, taking it in 24 minutes with a forehand winner on her third matchpoint.
Fifth seeded Serb Ana Ivanovic cruised past Switzerland's Patty Schnyder 6-0, 6-2 in a first round match.
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