Serena made to struggle

Serena Williams: yet to drop a set in three matches

Three-time champion Serena Williams reached the last 16 of the US Open with a 6-1 7-6 win over fourth-seed Victoria Azarenka of Belarus.

Williams, a 13-time Grand Slam winner, looked on course for an easy victory after racing through the first set.

She had looked in supreme form in her first two matches, dropping only three

games, and the early signs were ominous in the extreme for Azarenka.

The fourth seed was broken in her first service game when Williams powered away a backhand winner, and she promptly smashed down two aces to make it three games in a row.

Comeback

At 5-0 a 'bagel' looked in prospect, but Azarenka battled hard to come back from 0-30 and at least get on the board in the first set.

The second set was more competitive although it looked all over as Williams led 5-3.

But she wasted four match points - three of them from 0-40 on the Belarussian's serve - as Azarenka bravely took the match into a tiebreak.

At 5-5 in the breaker, the match could still have gone either way but it was the 28th-seeded Williams who edged it, as Azarenka netted a forehand to give Williams her fifth match point at 5-6.

She did not squander this one, pressurising Azarenka to send a forehand wide to remain on course for a quarter-final showdown against top seed Caroline Wozniacki.

Williams hit 39 winners including a dozen aces and may well feel that this was the perfect workout as she approaches the business end of the tournament.

Ivanovic awaits

Next up for Williams is Ana Ivanovic, who ended the run of United States teenager Sloane Stephens with a 6-3 6-4 victory in the night session on Arthur Ashe Stadium.

The Serb star needed just one hour and 10 minutes to see off the home favourite and reach the round of 16 at Flushing Meadows for the third time in her career.