Not much came easy for the 23-time Grand Slam winner against her plucky Dutch opponent, but her unwavering determination enabled her to survive a testing encounter.
The latest duel between the all-conquering Williams sisters will be their first since last year’s Australian Open final, which went the way of Serena.
Venus booked her spot in the round of 32 by overcoming Sorana Cirstea 6-3 6-4. The 37-year-old had a more leisurely afternoon in comparison to her younger sibling, however she was still required to recover from a break down in the second set.
Moments after Venus had departed a-filled-to-the-brim Court 1, Serena arrived to a rousing reception. The former world No.1 made a shaky start against her capable rival, and fell a break and 2-1 behind.
Mertens was superior in the early stages and fully merited her early lead – despite struggling with her first serve percentage. The 26-year-old then had an opportunity to move a double-break in front, but she netted a regulation backhand and the chance was gone.
The Dutchwoman had squandered a glorious chance to strengthen her grip on the opener, and she was broken back in the following game. It was a passage of play to forget for the No.29 seed – and it meant her good early work counted for nothing.
Despite the setback, Mertens remained upbeat and achieved a second break to move 4-3 ahead. A forehand return winner from the underdog did the damage, and her moment of inspiration was greeted with stunned silence.
Mertens eventually had the opportunity to close out the set behind her own deal, but she wilted under pressure. Williams achieved a crucial break with a well-placed backhand, and she was back in the hunt for the all-important opener.
Perhaps fittingly, the first set went to a tiebreak. It was closely-fought, but Williams snatched it by seven points to five. A double-fault from Mertens while trailing 3-2 did little to assist her cause.
It was a scratchy first set from the American. Wild misses often followed moments of trademark brilliance and it’s certainly going to take the 36-year-old time to find a level of consistency again.
The second set was equally tight, however Williams eventually got over the winning line after a nervy finale. She failed to apply the finishing touches at the first time of asking, but made no mistake on the second occasion.
Williams may have been agitated and bewildered by her decision-making at times, but her ability to elevate her level when it really counts remains unrivalled.
Earlier in the day, Elina Svitolina soared past Mona Barthel 6-4 6-3, Carla Suarez Navarro edged out Su-Wei Hsieh 6-4 2-6 6-3 and Julia Goerges thumped Natalia Vikhlyantseva 6-4 6-1.
Defending champion Elena Vesnina is also safely through after defeating Catherine Bellis 2-6 6-1 6-1.