(Reuters) - Madison Keys overcame some uncharacteristically poor serving and elevated her game late on to edge fellow American Sloane Stephens 7-6(6) 4-6 6-2 and advance to the semi-finals of the Charleston Open on Friday.
Both players struggled with their serves in a match that included 13 breaks but it was Keys who was able to cut down on the errors and step up the intensity in the third set, which she won when Stephens sent a backhand long.
The match on the green clay also featured a seesaw first set tie-breaker in which Keys jumped out to a 4-0 lead before Stephens stormed back with six straight points.
Stephens failed to convert either of the two set points she had on her serve and Keys unleashed a blistering cross court forehand winner to capture the hour-long first set.
"It's always tough to play a friend and it kind of shows at times like that," Keys said of the wild first set tie-breaker.
"It was up and it was down and it was just kind of crazy."
The win was a first for Keys over Stephens, who came out on top in their semi-final match up at last year's French Open and in the 2017 U.S. Open final.
Next up for Keys is either Puerto Rican Monica Puig or American Danielle Collins, who meet in final quarter-final.
Former world number one Caroline Wozniacki looked like she had fully recovered from the illness that sidelined her for two months earlier this year, playing nearly perfect tennis to down Maria Sakkari 6-2 6-2.
Wozniacki, the 2011 champion in Charleston, smacked 15 winners to just eight unforced errors and looked particularly sharp returning serve, where she moved briskly and attacked Sakkari's second serves.
The only hiccup came in the match's 16-minute final game, where Wozniacki needed five match points before she was able to break the young Greek's service again to win the tie.
The fifth-seeded Dane will be a heavy favourite in her semi-final showdown on Saturday against Petra Martic, who upset Belinda Bencic 6-3 6-4 in the morning match.
Wozniacki has never dropped a set to the Croatian in five career meetings.
(Reporting by Rory Carroll, editing by Nick Mulvenney)