Stephens, a former top 10 player now ranked 73rd, committed only 14 unforced errors. Her best showing at Wimbledon was in 2013, when she reached the quarterfinals.
Top-seeded Petra Kvitova had to dig deep to reach the second round of the Bad Homburg Open in beating Polish qualifier Katarzyna Piter 4-6, 6-1, 6-4.
Kvitova, who reached the semifinals at Roland Garros in 2012 and last year, next faces either Elena Vesnina or Belarussian qualifier Olga Govortsova.
Kvitova broke Muguruza's serve five times in a row to seal the first set and take a commanding lead in the second.
Novak Djokovic defeated Pablo Carreno Busta 4-6, 6-2, 6-3, 6-4 to make it to his 10th semi-final at the Roland Garros.
Kvitova normally dictates play with her big serve and forehand but she double-faulted multiple times when she was broken twice in the second set.
The seventh-seeded Czech player lost her only semifinal match at Roland Garros in 2012.
The sixth-seeded Williams served for the match at 40-0 but Ahn saved three match points. An ace gave her a fourth opportunity to win but she hit a shot into the net.
The event is scheduled to be played from May 26 to May 28 in the capital city of Czech Republic.
Ash Barty beat former No. 1 Garbine Muguruza, 6-1, 6-7, 6-2 Thursday to reach the semifinals of the Qatar Open.
The third-seeded Kvitova had been due to play Ekaterina Alexandrova in the quarterfinals. Kvitova said the unspecified illness had bothered her during Thursday's three-set win over Alison van Uytvanck.
Barty beat two-time Wimbledon champion Petra Kvitova 7-6 (6), 6-2 on Rod Laver Arena and will next play Sofia Kenin.
Milos Raonic's perfect record at Melbourne Park this year — four straight-sets victories, including a third-round win over No. 6 Stefanos Tsitsipas — is in for a serious quarterfinal test against a guy he's never beaten in nine attempts.
Serena Williams defeated Tamara Zidansek 6-2, 6-3 while Naomi Osaka and Ash Barty got the better of Zheng Saisai and Polona Hercog 6-2, 6-4 and 6-1, 6-4 respectively.
Naomi pulled it together quickly in a swirling breeze on Margaret Court Arena, though, and beat Zheng Saisai 6-2, 6-4 to advance to the third round of the Australian Open.
The American won nine of 10 games in her mid-match recovery against the tournament's 2011 champion to beat Kvitova 3-6, 6-2, 6-3.
Naomi Osaka defated Kiki Bertens 6-3, 3-6, 6-3 while Petra Kvitova beat qualifier Jennifer Brady 6-4, 6-2. Madison Keys got the better of fellow American Danielle Collins 6-4, 6-1.
Serena Williams came back from one-set down to defeat 83rd seed Vitalia Diatchenko. Petra Kvitova, however, had to pull out of the tournament with an injury.
The victory allows the 21-year-old Osaka to become the first tennis player from Japan to reach No. 1 in the rankings.
Petra Kvitova defeated Danielle Collins 7-6 (2), 6-0 while Naomi Osaka downed Karolina Pliskova 6-2, 4-6, 6-4 in the semi-finals to set up a summit clash against each other.
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