Serena Williams reached the semi final at a ninth consecutive major
Williams lost her first set of the tournament but came back to take the third
The 22-time Grand Slam winner beat fifth seed Simona Halep 6-2, 4-6, 6-3
By Stuart Fraser for MailOnline
Serena Williams came through her toughest test yet at this year's US Open as she beat Simona Halep in three sets to reach her 33rd Grand Slam semi-final.
Williams dropped her first set of the tournament but still proved too strong for fifth seed Halep with a 6-2, 4-6, 6-3 win in two hours and 14 minutes.
Williams said: 'I didn't have to be too amped up [to play Halep] because it's the quarter-finals and it's a tough player. I just knew I needed to be ready.
'I think that she obviously plays well, so it was definitely a jump [in difficulty].'
The 34-year-old American will now take on Czech Karolina Pliskova in the last-four on Thursday as she bids to retain her world No 1 ranking.
Angelique Kerber's run to the semi-finals means that Williams must reach the final to prevent her German rival from taking over the top spot. If the pair meet in the final, then Williams must win.
History is also at stake with Williams on the brink of breaking Steffi Graf's record of 186 consecutive weeks at world No 1.
Williams had a dominating 7-1 lead in the head-to-head record with Halep coming into the match and did not take long to send out a message to her opponent with three aces in the opening game.
Williams' aggression helped her ease to the first set in 33 minutes, hitting 13 winners to Halep's three and six aces to none.
Halep battled hard by saving seven break points early a lengthy second game of the second set and then broke serve herself for 2-1 as Williams double faulted.
Halep served for the second set at 5-4 and managed to scrape through a thrilling 12-minute game, saving five break points and then converting her fifth set point as Williams hit a backhand long.
But a poor game from Halep saw Williams break for 3-1 in the deciding set and she took charge from there, comfortably serving out the win in one of the best matches of the tournament so far.
Williams said: 'I feel great. I'm glad I got tested. The best part of it is I think her level really picked up in the second set but I had some opportunities that I didn't take.
'So what I gather from that is I really could have played better in that second set and maybe had an opportunity to win in straights. I think if anything, that's the biggest silver lining I take.'
