IT was, in his own words, a "big win" over a big-serving opponent.

Andy Murray pronounced himself highly satisfied after advancing to the ­quarter-finals of the Western & Southern Open with a hard-fought victory over John Isner yesterday.

The 2011 champion was almost faultless for most of the first set but the wheels came off in the tie-break as Isner took the initiative.

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Murray produced the only break of the match to take the second, but the third went to another tie-break and the eighth seed made his class count as he closed out the match 6-7 (3/7), 6-4, 7-6 (7/2).

The duo largely cancelled each other out in the first set, with Isner's big forehand return causing plenty of problems for Murray, but the Scot was easily outclassing his opponent at the net. Isner was making the early errors, finding the net every time Murray brought him short, but he produced a stunning backhand volley to win the fourth game.

The rest of the set went with serve and Murray secured a mini-break on just the second point before moving 3-1 ahead. But Isner came roaring back, his big returns proving unplayable off Murray's second serve, and took the tie-break 7/3.

The world No.9 could have been forgiven for being shellshocked, but he delivered the perfect response by breaking Isner in the first game of the second set.

Murray had two break points but Isner fought back before eventually succumbing to the first break of the match. The rest of the set went with serve, with Murray keeping his foe at bay with little trouble.

That pattern continued until the Briton double-faulted in the 11th game of the third to hand Isner match point, but once again his volleying let him down as he found the net.

Murray saw off another match point at 40-40, but then produced a sublime winner and an ace to take the game to a deciding tie-break.

And the former US Open and Wimbledon champion proved too strong as two good returns forced Isner to find the net. The American 11th seed finished with a double fault.

"It was a big win for me," Murray said. "He plays extremely well here. Returning his serve, especially his second serve, in these conditions is very challenging so I did well to fight back. I felt like I played a very good match. I served well, I moved well, I was hitting the ball clean from the back of the court and when I was challenged at the end of the third set I responded well, that's important.

"It's all about winning right now, I don't really care how I play, I just try and keep getting wins."

Roger Federer and Gael Monfils were in action in the early hours to decide who would play Murray next but the No.1 seed, Novak Djokovic, no longer stands in his way after the Serb was beaten 7-6 (8-6), 7-5 by Spain's Tommy Robredo.

World No.1 Serena Williams continued her march towards a maiden Cincinnati title, easing into the quarter-finals with a 6-2, 6-2 demolition of 13th seeded Italian Flavia Pennetta.

Second-seeded Romanian Simona Halep beat Lucie Safarova 6-4, 7-5.