The American was a little below par but Mertens, the No.28 seed, was inspired, serving well and taking the attack to Pegula throughout as she reached the last 16 for the third time.
Mertens sends Pegula packing
Belgian knocked out the No.3 seed to improve to 3-0 head-to-head against her
Story of the match
Mertens had won their two previous meetings, both on hard courts and Pegula had acknowledged she would need to play near her best to have a chance.
Unfortunately, she didn't and Mertens dominated from the start, before holding her nerve when things got tighter in the second set.
With another Belgian, Kim Clijsters, the former world No.1 and a mentor to Mertens throughout her career, watching in the stands, the No.28 seed began strongly, racing to a 5-0 lead.
A quarter-finalist last year, Pegula had looked good in her opening two matches but was struggling to win points on serve, thanks in part to the aggressive stance of Mertens on return.
Pegula found her range at last to get on the board with a love hold in the sixth game, finishing off with her first two aces. But she was not her usual calm self, her frustration obvious as the errors flowed.
Mertens held easily to take the opening set after just 26 minutes and when she broke Pegula in the first game of the second, it looked like she would cruise to victory.
But Pegula dug in and began to make an impression, finally breaking back when Mertens netted a backhand.
With the wind causing both players problems, games went with serve to 3-3. Great defence from Mertens had the crowd on their feet as she moved to 15-30 and then an overhit slice gave the Belgian two more break points. She only needed one as Pegula hit a forehand into the net to go ahead 4-3.
The eighth game was a mini classic. Pegula hit a nice drop shot on the opening point but Mertens returned the favour immediately. Pegula gave herself half a chance at 15-30 but her backhand again let her down as Mertens moved to game point.
Suddenly Pegula upped her aggression to get back to deuce and a poor drop volley gave Pegula a break back point. Again Mertens was equal to the task, putting away a smash to save it.
Pegula then produced one of her best points of the match, returning a bullet of a first serve and then threading a forehand pass for a winner. Mertens saved it with a fine point of her own, a brilliant drop shot gave Pegula a third chance but she couldn’t take it and Mertens finally held when the American’s forehand found the net.
That proved to be Pegula’s last chance. Serving to stay in the match, a forehand into the net handed Mertens two match points and she took her first opportunity, clinching victory as Pegula netted a forehand on the run.
Keys to the match
Pegula’s serve takes a hammering
Pegula can usually rely on getting a good percentage of points on her first serve but she found life tough throughout. She won just 50 percent of points on first serve, and only 39 percent on her second serve. Mertens was aggressive throughout, never allowing her to settle.
Mertens strong on break points
When Pegula did manage to work herself the odd chance, it was quickly blocked by the tenacity of Mertens. The Belgian saved five of the six break points she faced and took all five of her own break points.
Mertens dominating long rallies
When the rallies went long, Mertens was in her element. The No.28 seed won 13 of the 15 points when the rallies went to nine or more. For good measure, she also had the edge in the 0-4 and 5-8 categories too.
What the winner said
On the victory and playing on Court Philippe-Chatrier
"I was very happy to win in two sets. I think I played well. I was delighted to play on this amazing court. Thank you to everyone, it was amazing."
On being tested hard in the second set
"The first set was easy, it was amazing. During the second set at 4-3, it was really difficult. I was very happy to win that game. You never know how it can end. I was very happy to win that game and the match."
On being inspired by the likes of Clijsters and the Belgian fans
"Yes, what they did was amazing. I was very happy they were here. I was very happy to be here. Thank you very much to the public and the French. It’s like playing at home."