Second seed Gilles Simon was left to rue what could have been as he lost to Sam Querrey in the ATP Aegon Open quarter-finals.

A gutsy display from Querrey earned him a place in the Aegon Open semi-finals as he punished second seed Gilles Simon for squandering two match points.

The American found himself faced with a quarter-final exit as he trailed 6-5 and then 8-7 in a second set tie-break, but on both occasions he dug deep and eventually forced a decider.

During the third set, the 12th seed earned decisive back-to-back breaks before successfully serving out for a 5-7, 7-6 (10-8) 6-4 success over the Frenchman.

Awaiting Querrey in the last four is Alexandr Dolgopolov, who beat Lu Yen-hsun 6-4 4-6 7-6 (7-4) in the final game of Thursday's action at the ATP 250 grass-court tournament in Nottingham.

Also through to the semi-finals is Denis Istomin after he edged a thrilling three-set tussle to get the better of fourth seed Leonardo Mayer 6-3 6-7 (8-10) 7-6 (8-6).

Istomin, chasing his first ATP singles title and the lowest ranked player left in the field, forced the only break of the match in the fourth game of the opener as two tense tie-breaks proved decisive.

Match point went begging for Istomin in the second set before Mayer levelled the match at the third time of asking. It was then Mayer's turn to spurn match point as he led 6-5 in the tie-break at the end of the decider, only for Istomin to rattle off the next three points and set up a last-four meeting with Marcos Baghdatis.

The Cypriot, who dumped out top seed David Ferrer earlier this week, was on court first on Thursday and defeated Simone Bolelli 6-4 6-4.