Serena Williams after thrashing Kaia Kanepi

World number one and two-time defending champion Serena Williams reached the US Open quarter-finals on Monday with a 6-3, 6-3 win over Estonia’s Kaia Kanepi.

The five-time champion will face fellow 32-year-old Flavia Pennetta for a place in the semi-finals.

“It’s my first quarter-final of the year at the majors — at last I did it,” said the American, who had fallen in the fourth round in Australia, the third round at Wimbledon and suffered a second round French Open exit.

“It was a tough match today. Kaia hits the ball very hard and moves the ball around really well. I wanted to stay relaxed and told myself that whatever happens, Serena, you’re still in the doubles.”

In contrast,  Canadian poster girl Eugenie Bouchard, struggling in the heat and humidity, was knocked out.

Seventh seeded Bouchard was at the centre of a medical drama on Louis Armstrong court when she had to have her blood pressure and temperature taken in her 7-6 (7/2), 6-4 defeat to Russian 17th seed Ekaterina Makarova.

Wimbledon runner-up Bouchard called the medical timeout at 2-3 down in the second set when she was obviously struggling in the 31-degree heat (87F) and humidity running at 60 percent.

She slipped 4-2 down, broke back for 4-3 but the left-handed Makarova was too strong for the ailing Canadian as she booked a spot in the last-eight for the second successive year.

Bouchard’s defeat ended her run of having reached at least the semi-finals at all the majors this year and also means that for the first time since 1977, the four Grand Slam women’s championship matches will have featured eight different players.