A poor service game for Kristie Ahn just when she was serving for the first set. We're almost at an hour and there's still a lot of juice in set one. Five-all back on serve.

Naomi Cavaday
BBC Radio 5 Live tennis commentator

Just as we thought Serena had got a grip on it and ramped it up, she's gone back into rallying again. It is Ahn who is being more aggressive.

The first break point goes as a backhand is long. 15-40

Three break points for Serena Williams.

Image source, Reuters

There were two surprises in the early afternoon matches, with 14th seed Fabio Fognini and Canada's 19th seed Felix Auger-Aliassime dumped out of the competition.

Italian Fognini, 33, suffered a 7-5 3-6 7-6 (7-1) 6-0 defeat by Kazakh former top-50 player Mikhail Kukushkin. And Auger-Aliassime, 20, who reached the fourth round at Flushing Meadows, lost 7-5 6-3 6-3 to Japan's Yoshihito Nishioka.

Ahn is serving for the set but it's not going well so far. 0-30.

A sweeping forehand from the baseline from Williams, but there is too much on it and out it goes. Williams had to work hard to get back in the set, but her opponent has just broken immediately afterwards.

Ahn now to serve for set one.

Williams has just broken Ahn, but is now 0-40. Chances for Ahn to instantly retake control of this set.

British qualifier Liam Broady has gone down an early break against Jiri Vesely, overhitting a forehand to allow the Czech to go 3-1 up.

Very early days though.

A scream from Williams as a winner gives her break point number four. Ahn's first serve is no good, another rally follows before Williams cranks up the power. A blistering forehand pulls Ahn out of position and a backhand finishes it off.

Williams breaks. Back on serve.

Miles Maclagan
Former British Davis Cup player on BBC Radio 5 Live Sports Extra

What a point! How did Ahn get out of that one? Great defensive skills. Serena is scratching her forehead.

Ahn tore around the court, that backhand towards the end where she shaped up for a dropshot and pushed it deep caught Williams off balance.

A wonderful point. Ahn looks finished as she sends one high from the baseline, but it lands right on the line and the point continues. It lasts 19 shots and Ahn takes it, much to Williams bafflement.

Ahn, sent wide, has a double-handed backhand down the line but it hits the net and Williams has a third break point chance. But a solid forehand from Ahn finds the corner. Three chances for Williams, all missed.

But here's another one after a brutal no-nonsense forehand return.

We're up to seven minutes for this game and this is going to be a tough take for whoever loses it. If Ahn claims it then she will be one game from the opening set.

A second chance to get us back on serve after Kristie Ahn with a slow looping forehand cross-court shot sends it wide.

The opportunity doesn't last long though as Ahn saves it with an ace.

Naomi Cavaday
BBC Radio 5 Live tennis commentator

Serena is just starting to get a little louder and ramping things up.

When they played at the US Open, Williams stood way up the court, really trying to threaten and crush Ahn's serves.

A chance for Williams to break back. But she's furious with herself as she misjudges a return.

This is Serena's outfit today...

Image source, Getty Images

...and Australia's Daria Gavrilova, who knocked out Ukraine's 24th seed Dayana Yastremska yesterday, thinks she has seen it before...

A fine serve out wide helps Williams to take her next service game. She has only had problems in one service game, but unfortunately for her that's the one she lost her serve in.

Kristie Ahn edges closer to the first set with a hold to 15. The final point is a slow second serve, which looks like an inviting present but Williams can't take the gift and Ahn's lead moves to 4-2.