Venus Williams scored a straight sets upset of 16th seed Veroninka Kudermetova at the ATP/WTA Cincinnati Open on Monday, bagging her first win over a top 20 player in four years.

The 43-year-old American veteran, who was granted a wild card into this week's tournament, rallied superbly to polish off Kudermetova 6-4, 7-5.

The seven-time Grand Slam singles champion, whose career has spanned four different decades, showed great heart to recover from 4-1 down in the opening set before overturning a 5-1 deficit in the second to seal victory.

"I love this game, it's what I do," Williams said in on-court remarks after her victory against the Russian world number 14.

"The last few years have been difficult in terms of injuries," Williams added. 

"I want to be able to be out here and be strong, and be myself and that's important to me, and I'm trying to get there."

Williams, a semi-finalist at Cincinnati in 2012, saw her match interrupted by rain before she completed her victory. 

In other women's matches on Monday, 2017 US Open champion Sloane Stephens marked her 12th appearance with a 7-5, 6-2 defeat of Italian Elisabetta Cocciaretto.

"This (summer) series so far has been tough. D.C., there was rain. Last week, Montreal was rain. This week again, rain," Stephens said.

In the men's draw, Felix Auger-Aliassime won a battle of injury comeback competitors, keeping former Wimbledon finalist Matteo Berrettini winless on the Tour since March with a 4-6, 6-2, 6-3 victory.

Auger-Aliassime, seeded 12th, missed his home Masters last week in Toronto with his injury recovery not yet complete.

Victory snapped a five-match loss streak stretching to the week before Roland Garros in May.

Finn Emil Ruusuvuori beat Corentin Moutet 6-4, 6-2. 

Top seed Carlos Alcaraz will open on Wednesday after a bye against Jordan Thompson, who put out 2013 finalist John Isner 7-6 (7/5), 7-6 (7/2). 

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